Textual criticism (or lower criticism) is a branch of literary criticism that is concerned with the identification and removal of transcription errors in the texts of manuscripts. Ancient scribes made errors or alterations when copying manuscripts by hand. Given a manuscript copy, several or many copies, but not the original document, the textual critic seeks to reconstruct the original text (the archetype or autograph) as closely as possible. The same processes can be used to attempt to reconstruct intermediate editions, or recensions, of a document's transcription history. The ultimate objective of the textual critic's work is the production of a "critical edition" containing a text most closely approximating the original.
http://en.wikipedia.org/wiki/Textual_criticism