A piece of software used for tagging texts, i.e. labelling words with the part-of-speech appropriate to the context. A POS-Tagger, for instance, will usually work in tandem with a syntactic- parser.
In the context of linguistics technology, a tagger is a program that labels words in a document according to their role in sentences. For example, words may be classified according to their part of speech or other linguistic properties.
As opposed to "writer"; this term is usually used to refer to those who only do tags and throwups and who never piece. Some taggers seem to like more destructive methods such as scribers and sandpaper in addition to markers and paint. Some taggers do get interested in piecing, some don't. Taggers who never piece are sometimes called "scribblers" by more experienced, piecing writers.