grep lets you search through multiple text files at once.
I recently removed a feature from a project and wanted to make sure I removed all references to it. With grep it was easy:
grep -r picture *
This searched all the files in my current directory as well as all the files in each subdirectory, and printed the filename and line where the word "picture" appeared.
Then I went in and changed the 2 files that needed modifying.
Had I wanted to just search the current directory (without the subdirectories) I could have done:
grep picture *.*
This would have searched through all the files in the current directory.
Had I wanted to search in just 1 specific file:
grep picture filename.txt
Had I wanted to do a case insensitive search:
grep -ir picture *
Had I wanted to exclude a directory and done a case insensitive search:
grep -ri "searchstring" --exclude="*\.svn*" *
(grep uses regular expressions)