Tabs are numbers instead of musical notes. It is for people who can't read music (like me).

Here is an example. http://www.guitaretab.com/i/ivy/260936.html

Each line corresponds to a respective string, and each number corresponds to a different fret. All numbers stacked in a vertical line are played at the same time (strummed). There is no way to read the tempo, other than if there are additional notes, or you just playing it how it sounds.