When studying the New Testament, it is important to consider how these writings were first produced, how they were copied and re-copied by scribes, what errors or corruptions may have been introduced, and how the modern scholar or text critic arrives at what she thinks is closest to the original Greek document produced in the first century. The term textual criticism refers to the modern scholarly practice of determining, as closely as possible, the original text of some ancient writing by sifting through the manuscripts of that writing that have survived. In the case of the New Testament there are over 5,000 manuscripts or fragments to sift through. There is an excellent site on ancient writing techniques, scribal practices, and textual criticism by Timothy Seid (professor at Earlham School of Religion) which I highly recommend as an introduction to these matters:
Interpreting Ancient Manuscripts
The NTGateway also supplies several other useful links on textual criticism.