grep is to find what sed is to replace.
Say you noticed a few typos in a file. Say you typed imposible instead of impossible a few times in a file named "stuff.txt".
You can fix all occurrences using this one-liner:
sed 's/imposible/impossible/g' stuff.txt
Often it's more helpful to sed across multiple files.
sed -i 's/1.0.1/"1.0.1"/g' *.php
That's one command I used to add quotes around 1.0.1, which I forgot to do. Alas, it's not recursive.