In most shells, aliases are an easy way to shorten a long command line or do a short series of commands. Section 29.2 through Section 29.10 cover C shell aliases. Section 29.4 through Section 29.14 cover aliases in bash, pdksh, and zsh.
All except the oldest Bourne-type shells have shell functions (Section 29.11), which are explained in Section 29.11 through Section 29.13. These are a cross between aliases and shell scripts. They're good both for shortening command lines and for running a short or long series of commands.
—JP and SJC