Redirection

Sending one form of output to a form of input other than that which it was originally intended. Common operands include: >, >>, <, <<, and the ever popular |.

>  sends output somewhere (usually a file)
>> appends output somewhere (without overwriting it)
<  sends output the other way
<< appends output the other way
|  sends output to a program (frequently, a system command like grep)

If you're using the bash shell, because you're not a sadist and like useful shells, you also have some special options available to you: you can redirect standard input, standard output and standard error messages.