difference when using 'distinct on'

Howdy:

Can someone tell what the difference (and why
you would use it) is between the following:

[snip]
select distinct on (col_1, col_2),
col_1,
col_2,
col_3
from t_table

--

select distinct
col_1,
col_2,
col_3
from t_table
[/snip]

In the first example, is it just getting
the unique rows for the first two columns?

Thanks!

-X