| This could probably go into "General Instruction" as well. It's up to the mods if they want it there or here or both.
Anyway, most TAB just shows you which fret to use, not the note duration. However, a lot of commercial TABS (those in books and on pay web sites) that also have bass staff will indicate duration and rhythm.
For example, in some score books, you'll have the bass staff plus the TAB, and the TAB will indicate fingerings and duration. A whole note F for example will be written as a 3 within a circle on the D string. A half note F would be written as a 3 within a circle with a stem on the D string.
Unfortunately, there is no standardization among TABs. Much of the TABs found on the 'net on the free TAB sites don't have duration or rhythms, though some people try to indicate this with a text section at the top or bottom of the TAB explaining things to some extent. | 
