You may have used chown command to change the ownership of files or directories like below.

$ ls -l file
-rw-r--r--  1 root root  675 2013-09-23 23:23 file
$ sudo chown foo file
$ ls -l file
-rw-r--r--  1 foo root  675 2013-09-23 23:23 file

But this is not good because group of file is still root. Most case, you want to set it the default(login) group of the user.

$ id foo
uid=1010(foo) gid=1009(foo) groups=1009(foo)

In this case, you want to set it foo. For that, I used a command below.

$ chown foo:foo file
$ ls -l file
-rw-r--r--  1 foo foo  675 2013-09-23 23:23 file

Not bad. But you have to know the default group of the user. It is not always same as the user name.

So, here is the best way. Just add : after the user name.

$ sudo chown foo: file
$ ls -l file
-rw-r--r--  1 foo foo  675 2013-09-23 23:23 file

Wow! This is super easy. See also man chmod

… If a colon but no group name follows the user name, that user is made the owner of the files and the group of the files is changed to that user's login group. …