The following is taken from the Preface of the New King James Version (NKJV) of the Bible, pages v-vi. I find explanations like these regarding the trustworthiness of the Scriptures very helpful, and so I am copying this excerpt here:
There is more manuscript support for the New Testament than for any other body of ancient literature. Over five thousand Greek, eight thousand Latin, and many more manuscripts in other languages attest the integrity of the New Testament. There is only one basic New Testament used by Protestants, Roman Catholics, and Orthodox, by conservatives and liberals. Minor variations in hand copying have appeared through the centuries, before mechanical printing began about A.D. 1450.
Some variations exist in the spelling of Greek words, in word order, and in similar details. These ordinarily do not show up in translation and do not affect the sense of the text in any way.
Other manuscript differences such as omission or inclusion of a word or a clause, and two paragraphs in the Gospels, should not overshadow the overwhelming degree of agreement which exists among the ancient records. Bible readers may be assured that the most important differences in English New Testaments of today are due, not to manuscript divergence, but to the way in which translators view the task of translation: How literally should the text be rendered? How does the translator view the matter of biblical inspiration? Does the translator adopt a paraphrase when a literal rendering would be quite clear and more to the point? The New King James Version follows the historic precedent of the Authorized Version in maintaining a literal approach to translation, except where the idiom of the original language cannot be translated directly into our tongue.
The King James New Testament was based on the traditional text of the Greek-speaking churches, first published in 1516, and later called the Textus Receptus or Received Text. Although based on the relatively few available manuscripts, these were representative of many more which existed at the time but only became known later. In the late nineteenth century, B. Westcott and F. Hort taught that this text had been officially edited by the fourth-century church, but a total lack of historical evidence for this event has forced a revision of the theory. It is now widely held that the Byzantine Text that largely supports the Textus Receptus has as much right as the Alexandrian or any other tradition to be weighed in determining the text of the New Testament. Those readings in the Textus Receptus which have weak support are indicated in the center reference column as being opposed by both Critical and Majority Texts (see "Center-Column Notes").
Since the 1880s most contemporary translations of the New Testament have relied upon a relatively few manuscripts discovered chiefly in the late nineteenth and early twentieth centuries. Such translations depend primarily on two manuscripts, Codex Vaticanus and Codex Sinaiticus, because of their greater age. The Greek text obtained by using these sources and the related papyri (our most ancient manuscripts) is known as the Alexandrian Text. However, some scholars have grounds for doubting the faithfulness of Vaticanus and Sinaiticus, since they often disagree with one another, and Sinaiticus exhibits excessive omission.
A third viewpoint of New Testament scholarship holds that the best text is based on the consensus of the majority of existing Greek manuscripts. This text is called the Majority Text. Most of these manuscripts are in substantial agreement. Even though many are late, and none is earlier than the fifth century, usually their readings are verified by papyri, ancient versions, quotations from the early church fathers, or a combination of these. The Majority Text is similar to the Textus Receptus, but it corrects those readings which have little or no support in the Greek manuscript tradition.
Today, scholars agree that the science of New Testament textual criticism is in a state of flux. Very few scholars still favor the Textus Receptus as such, and then often for its historical prestige as the text of Luther, Calvin, Tyndale, and the King James Version. For about a century most have followed a Critical Text (so called because it is edited according to specific principles of textual criticism) which depends heavily upon the Alexandrian type of text. More recently many have abandoned this Critical Text (which is quite similar to the one edited by Westcott and Hort) for one that is more eclectic. Finally, a small but growing number of scholars prefer the Majority Text, which is close to the traditional text except in the Revelation.
In light of these facts, and also because the New King James Version is the fifth revision of a historic document translated from specific Greek texts, the editors decided to retain the traditional text in the body of the New Testament and to indicate major Critical and Majority Text variant readings in the footnotes. Although these variations are duly indicated in the footnotes of the present edition, it is most important to emphasize that fully eighty-five percent of the New Testament text is the same in the Textus Receptus, the Alexandrian Text, and the Majority Text.