The Critical Text is a concept, a theoretical Greek text of the New Testament constructed from various sources. The notion is to draw from ancient Greek manuscripts and their variants to construct a text of what the editors believe is the most accurate wording.
In 1881 B. F. Westcott and F. J. A. Hort printed their reconstructed New Testament, which became known as the Critical Text. They placed special emphasis on two 4th-century manuscripts, the Codex Vaticanus and the Codex Sinaiticus. This Critical Text (with its updates and revisions) has become the most popular Greek text today. Its early form was the basis for the English Revised Version and American Standard Version translations. More recently it has been the basis for the Revised Standard Version, English Standard Version, the New International Version, as well as most other modern translations of the Bible.
Merrill M. Parvis (University of Chicago Divinity School, Emory University, et al.) explains, "We have reconstructed text-types and families and sub families and in so doing have created things that never before existed on earth or in heaven. We have assumed that manuscripts reproduced themselves according to the Mendelian law. But when we have found that a particular manuscript would not fit into any of our nicely constructed schemes, we have thrown up our hands and said that it contained a mixed text." (Parvis, in "The Nature and Task of New Testament Textual Criticism," The Journal of Religion, XXXII (1952), 173, as quoted by Wilbur N. Pickering in The Identity of the New Testament Text II, page 21)
The gist is this. The Critical Text as constructed had no historical existence. It is a combination of variant readings from various manuscripts. Its existence as a single text came to fruition in the mind of the redactors. Only after it was put together on paper did it become an historical reality. In contrast to the Critical Text, the Byzantine text-form demonstrably existed for over 1,000 years and has been in constant usage by the Greek-speaking church during that period. (See, for example, New Testament Textual Criticism: The Case for Byzantine Priority by Maurice A. Robinson; "...the Byzantine Textform is the form of text which is known to have predominated in the Greek-speaking world from at least the fourth century until the invention of printing in the sixteenth century.")