A guitar tab is read as a score: for each line and number corresponds to a note.
In particular, the classic notes are replaced by numbers that correspond to the key and to press the rope to hit. For this we have 6 lines, corresponding to 6-string guitar! Clearly, for the duration of the notes refers to the traditional notation.
An example might be this: (see picture above)
Yes, yes, it is ... Sweet Child O'Mine
From the top we find the tuning of the instrument (here it is 1 / 2 tone below), the first note is G at the 12th fret, third string. Each key is simply a number and a note, and virtually the tabs are readable even for those unfamiliar with classical music writing.
There are several variations of the tab, but the most common are those for Guitar Pro, a must-have software!