Ah, the example on the previous panel brings up a couple more points! First, the apropos command is exactly equivalent to man -k. (In fact, I'll let you in on a little secret. When you run man -k, it actually runs apropos behind the scenes.) The second point is the makewhatis command, which scans all the man pages on your Linux system and builds the database for whatis and apropos. Usually this is run periodically by root to keep the database updated:

# makewhatis

For more information on "man" and friends, you should start with its man page:

$ man man