Textual Criticism is the science of taking the thousands of Biblical manuscripts and reconstructing the original text which the authors wrote. In spite of the overwhelming agreement of the manuscripts of the New Testament and the Old Testament, there are some variations in some sentences. The result of textual criticism is that we have an extremely high confidence that our modern Bible is truly the same document that the original authors wrote nearly 2000 years ago.
Links to information and images about Biblical (and other ancient) texts are here.