Guitar tabs are useful for learning which notes to play if you can't learn a song by ear and you don't read music. Keep in mind that tabs only show you where to place your fingers on the fretboard and in what order the notes are played. They do not show you exactly WHEN to play the notes or how long to pause in between them. Tabs assume you already know how a song goes. That is, you can at least hum the guitar lick or riff in question.
Below you can see the tab for the intro to the song 'Smoke On The Water' by Deep Purple. The tab shows that you play 2 notes at a time, 1 on the G string and the other on the D string. Note that the neck is shown upside down with the thinnest string (High E) at the top. The numbers represent the frets you play to hear the correct notes for the riff. 0 (zero) means it is an open string. 3 means the 3rd fret, 5 is the 5th fret and so on. The '\' between the 6 and 5 means you slide down from the 6th fret to the 5th fret on those strings.
E |----------------------------------------------------------------| B |----------------------------------------------------------------| G |-0---3---5-----0---3---6-\-5-----0---3---5---3---0-| D |-0---3---5-----0---3---6-\-5-----0---3---5---3---0-| A |----------------------------------------------------------------| E |----------------------------------------------------------------|