And of course if you see more than one note at once, it's a chord:

Gmaj
E---3--
B---0--
G---0--
D---0--
A---2--
E---3--

A string is played open if 0 is indicated. So in this example, the lower two strings aren't played "open", they're just skipped.

Gmaj
E---3--
B---0--
G---0--
D---0--
A------
E------