A parser is a routine or program that analyses the structure of some input according to a formal grammar [L1 ]. Examples would be various XML parsers, a BibTeX parser, and the parser for Tcl itself.
Almost all programs include parsers - at least if they read a file and process it to perform a (possibly useful) action. Excel, Word, OpenOffice, Photoshop and so on all contain parsers.
Now-a-days parsers are most often generated directly from grammar specifications instead of written by hand. Applications for doing so are called parser generators.
See also Syntax parsing in Tcl, LogParser, grammatical analysis, parsetcl, Scripted Parsing.