apropos : apropos searches for a keyword in some internal database(s) and displays a list of commands related to the searched keyword. This is helpful when you don't know the exact name of a command or have no clue of the command to perform certain task. You can also google for it, but you have this additional choice.
[root@devel2 ~]# apropos mail
aliases (5) - aliases file for sendmail
clamav-milter (8) - milter compatible mail scanner
fetchmail (1) - fetch mail from a POP, IMAP, ETRN, or ODMR-capable server
formail (1) - mail (re)formatter
hesiod_free_postoffice [hesiod_getmailhost] (3) - Hesiod functions for retrieving user to postoffice mappings
hesiod_getmailhost (3) - Hesiod functions for retrieving user to postoffice mappings
htnotify (1) - sends email notifications about out-dated web pages discovered by htmerge
logrotate (8) - rotates, compresses, and mails system logs
mail (1) - send and receive mail
mailaddr (7) - mail addressing description
mailcap (4) - metamail capabilities file
mailq (1) - print the mail queue
mailstats (8) - display mail statistics
makemap (8) - create database maps for sendmail
mutt (1) - The Mutt Mail User Agent
muttrc (5) - Configuration file for the Mutt Mail User Agent
newaliases (1) - rebuild the data base for the mail aliases
Ctrl +r : When pressing this in a shell you will filter you history of typed commands by the letters you type after pressing the keys.
I typed here "apr" after pressing ctrl+r and it gave me "man apropos".
(reverse-i-search)`apr': man apropos
If you type Ctrl+r again it will move to the next match for the typed letter(s).