An alias is a (usually short) name that the shell translates into another (usually longer) name or command. Aliases allow you to define new commands by substituting a string for the first token of a simple command. They are typically placed in the ~/.bashrc (bash) or ~/.tcshrc (tcsh) startup files so that they are available to interactive subshells. If you type on regular terminal, it will be gone when you close the terminal. If you set frequently using command (which uses pipe and redirection), it will be so productive.
$ alias ll ='ls -al'