A simple, but useful, example is the rm command. By default this removes a file without any questions being asked, but we might prefer it to check that we really want to remove each file. Looking at the man page you can see that we want rm -i. We could make sure that this was the default by setting up and alias:
[phasnip@mijpnb1 ~]$ alias rm rm -i [phasnip@mijpnb1 ~]$ rm junk rm: remove `junk'? yes
The first argument is what we want to call the alias, and the second is what we want it to do. Typing alias without any arguments lists the aliases that have been set, and typing it with an existing alias name will show the definition of that alias. To remove an alias just use the unalias command.
[phasnip@mijpnb1 ~]$ alias ls ls --color=tty rm (rm -i)
When you set an alias it is only set in the current terminal window on the current computer, it will not propagate to any other terminal windows you may have open or that you open subsequently. To make sure this alias was always set, you would have to put it into one of your configuration files.