At the heart of the matter, being Reformed means holding to a Reformed Confession such as the Westminster Confession or the Three Forms of Unity (The Belgic Confession, the Canons of Dort, and the Heidelberg Catechism).  Historically Presbyterian churches hold to the former, while Reformed churches such as those in the URCNA hold to the latter.  The Three Forms are about 90 years older than the Westminster Standards, and are a summary of continental Reformed theology, while the Westminster Standards are a summary of Reformed theology as taught in Britain circa 1648.  I might add that any church that calls itself 'reformed' but doesn't hold to one of these standards or doesn't participate in a denomination that does is not 'reformed.'  There's really no such thing as an independent Reformed church.  It takes more than one pastor to reach complex theological and discliplinary rulings on matters, and a pastor's accountability to the Word simply doesn't happen in a stand-alone church.

There are several Reformed distinctives found in these confessions, most notably infant baptism, a certain view of the Lord's supper, and the doctrines of grace, but all these derive from the covenant theology that these confessions use.  Take, for example, Chapter VII of the Westminster Confession, entitled God's Covenant with Man: 

I. The distance between God and the creature is so great, that although reasonable creatures do owe obedience unto him as their Creator, yet they could never have any fruition of him, as their blessedness and reward, but by some voluntary condescension on God's part, which he hath been pleased to express by way of covenant.

II. The first covenant made with man was a covenant of works, wherein life was promised to Adam, and in him to his posterity, upon condition of perfect and personal obedience.

III. Man by his fall having made himself incapable of life by that covenant, the Lord was pleased to make a second, commonly called the covenant of grace: wherein he freely offered unto sinners life and salvation by Jesus Christ, requiring of them faith in him, that they may be saved, and promising to give unto all those that are ordained unto life, his Holy Spirit, to make them willing and able to believe.

IV. This covenant of grace is frequently set forth in the Scripture by the name of a testament, in reference to the death of Jesus Christ, the testator, and to the everlasting inheritance, with all things belonging to it, therein bequeathed.

V. This covenant was differently administered in the time of the law, and in the time of the gospel: under the law it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all fore-signifying Christ to come, which were for that time sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation, and is called the Old Testament.

VI. Under the gospel, when Christ the substance was exhibited, the ordinances in which this covenant is dispensed, are the preaching of the Word, and the administration of the sacraments of Baptism and the Lord's Supper; which, though fewer in number, and administered with more simplicity and less outward glory, yet in them it is held forth in more fullness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not, therefore, two covenants of grace differing in substance, but one and the same under various dispensations.

 This is covenant theology in a nutshell.  Add to this the next chapter, entitled Christ the Mediator, and the covenant of redemption comes partially into focus, admittedly not very clearly:

I. It pleased God, in his eternal purpose, to choose and ordain the Lord Jesus, his only-begotten Son, to be the Mediator between God and men, the prophet, priest, and king; the head and Savior of the Church, the heir or all things, and judge of the world; unto whom he did, from all eternity, give a people to be his seed, and to be by him in time redeemed, called, justified, sanctified, and glorified.

.... 

So there are three features of covenant theology: the covenant of redemption, whereby the Godhead agreed to redeem man before the foundation of the world, the covenant of works, whereby Adam, our federal head, was commanded by God to obey, and the covenant of grace, whereby God elected man unto salvation through Christ.  These covenants form the structure of the Bible, and are therefore the only authoritative prologemena, or a basis for interpreting Scripture.  Scripture interprets Scripture, so the basis of Scripture forms the basis for its interpretation.  From there, all doctrine is derived.  It's pointless to have a discussion about the truth or falsehood of the doctrines of grace, infant baptism, and the like unless this foundation is first agreed upon. 

There are several streams of Protestant Christianity floating out there at this time, a few of which claim to be 'reformed' or have allowed that label applied to them.  Case in point are reformed Baptists.  The London Confession of 1689 is almost an exact rip-off of the Westminster Confession, except for the most crucial point: it's understanding of covenant theology.  Chapter VII is entitled simply God's Covenant:

Paragraph 1. The distance between God and the creature is so great, that although reasonable creatures do owe obedience to Him as their creator, yet they could never have attained the reward of life but by some voluntary condescension on God's part, which He hath been pleased to express by way of covenant.1
1 Luke 17:10; Job 35:7,8

Paragraph 2. Moreover, man having brought himself under the curse of the law by his fall, it pleased the Lord to make a covenant of grace,2 wherein He freely offers unto sinners life and salvation by Jesus Christ, requiring of them faith in Him, that they may be saved;3 and promising to give unto all those that are ordained unto eternal life, His Holy Spirit, to make them willing and able to believe.4
2 Gen. 2:17; Gal. 3:10; Rom. 3:20,21
3 Rom. 8:3; Mark 16:15,16; John 3:16;
4 Ezek. 36:26,27; John 6:44,45; Ps. 110:3

Paragraph 3. This covenant is revealed in the gospel; first of all to Adam in the promise of salvation by the seed of the woman,5 and afterwards by farther steps, until the full discovery thereof was completed in the New Testament;6 and it is founded in that eternal covenant transaction that was between the Father and the Son about the redemption of the elect;7 and it is alone by the grace of this covenant that all the posterity of fallen Adam that ever were saved did obtain life and blessed immortality, man being now utterly incapable of acceptance with God upon those terms on which Adam stood in his state of innocency.

 See how much shorter it is?  Baptists and Presbyterians don't agree on infant baptism and the Lord's supper because they don't agree on covenant theology.  The arguments for infant baptism and the Reformed view of the Lord's supper are primarily covenantal.  Take away the robustness of covenant theology, and there is no argument.  The Reformed believe that not only is the Lord's supper a memorial (as the Papists believe) but it's also a means of grace and a sign and seal.  Here's Part I of WCF XXIX:

Our Lord Jesus, in the night wherein he was betrayed, instituted the sacrament of his body and blood, called the Lord's Supper, to be observed in his Church unto the end of the world; for the perpetual remembrance of the sacrifice of himself in his death, the sealing all benefits thereof unto true believers, their spiritual nourishment and growth in him, their further engagement in and to all duties which they owe unto him; and to be a bond and pledge of their communion with him, and with each other, as members of his mystical body.

The supper of the Lord Jesus was instituted by him the same night wherein he was betrayed, to be observed in his churches, unto the end of the world, for the perpetual remembrance, and showing to all the world the sacrifice of himself in his death,1 confirmation of the faith of believers in all the benefits thereof, their spiritual nourishment, and growth in him, their further engagement in, and to all duties which they owe to him; and to be a bond and pledge of their communion with him, and with each other.2
1 1 Cor. 11:23-26
2 1 Cor. 10:16,17,21

 Where does the difference in wording come from?  Chapter XXVII of WCF and XXVIII of the London Confession, its analog.  From WCF XXVII:

I. Sacraments are holy signs and seals of the covenant of grace, immediately instituted by God, to represent Christ and his benefits, and to confirm our interest in him: as also to put a visible difference between those that belong unto the Church, and the rest of the world; and solemnly to engage them to the service of God in Christ, according to his Word.

II. There is in every sacrament a spiritual relation, or sacramental union, between the sign and the thing signified; whence it comes to pass that the names and effects of the one are attributed to the other.

III. The grace which is exhibited in or by the sacraments, rightly used, is not conferred by any power in them; neither doth the efficacy of a sacrament depend upon the piety or intention of him that doth administer it, but upon the work of the Spirit, and the word of institution, which contains, together with a precept authorizing the use thereof, a promise of benefit to worthy receivers.

IV. There be only two sacraments ordained by Christ our Lord in the gospels, that is to say, Baptism and the Supper of the Lord: neither or which may be dispensed by any but a minister of the Word, lawfully ordained.

V. The sacraments of the Old Testament, in regard of the spiritual things thereby signified and exhibited, were, for substance, the same with those of the New.

 and the XXVIII of the London Confession:

Paragraph 1. Baptism and the Lord's Supper are ordinances of positive and sovereign institution, appointed by the Lord Jesus, the only lawgiver, to be continued in his church to the end of the world.1
1 Matt. 28:19,20; 1 Cor. 11:26

Paragraph 2. These holy appointments are to be administered by those only who are qualified and thereunto called, according to the commission of Christ.2
2 Matt. 28:19; 1 Cor. 4:1

See how much shorter it is?  See the lack of mention of the covenant of grace? So Baptists have a different (and in my view faulty) understanding of the Lord's supper (and infant baptism, though I haven't covered it) because Baptists have a much shallower understanding of covenant theology.  This understanding continues even with the 'reformed' Baptists of today, and are therefore not Reformed.  They're Baptists, just as they were when they wrote their own confession in 1689.