{nomencl} is for questions regarding the use of the nomencl package to create nomenclatures. Related packages: glossaries, acronym.
The nomencl package can be used to create nomenclatures. A similar package is glossaries, which has more configuration options and is applicable in a wider range of issues, however.
If the jobname is, say, foo.tex a compilation generates foo.nlo as helper file -- this must be processed after compilation with
makeindex -s nomencl.ist foo.nlo -o foo.nls
after that, a second compilation is necessary, as usual with index-related features.