Linuxgangster.org (??) has a good article up on the powerful grep command. GREP is one of the most useful command line tools in linux (really, there are so many it’s hard to choose, but this is one I use more frequently than most others…) grep can stand on it’s own to look for a term in many files (for example)
grep soughtafterterm *.txt
it will display which line numbers and which files it appears in.