Question about COPY command

["Josh Harrison" <joshques@xxxxxxxxx>]

Hi,
A basic question about the COPY command syntax....
This is the syntax in the postgres manual. 

COPY tablename [ ( column [, ...] ) ]
    FROM { 'filename' | STDIN }
......
.....

What is the difference between copying from 'filename' and copying from 'stdin' ??? 

Thanks
Josh