I don't make great use of all the gnu tools, but they sure come in handy some times.
Basic use is searching a file for a particular term, and grep spitting out each line that term occurs in.
grep [term] [file]
Search all files in current folder for a term.
grep -r -n -i [term] .
With the E flag you can use the usual regular expressions. So searching for either of two terms:
grep -E '[term1]|[term2]' [file]
Otherwise there's also a utility called egrep.
NB: [file]] doesn't even have to be a file, you can use wildcards and things there too.
There's probably a better way to do this, but to search for instances with all terms I use a pipe, like:
grep '[term1]' [file] | grep '[term2]'