Piano tabs, strictly speaking, are a form of sheet music that notates the music by showing where the fingers are to be placed instead of the notes themselves; the form is also known as tablature and is frequently used for guitar, as well. Technically, there is no such thing as piano tab. Tablature, though vast in its capabilities, is only intended for fretted string instruments, and while piano is certainly a string instrument, it isn't fretted. The term piano tab is actually used to refer to fingering charts (charts often used in instruction to aid in the correct placement of the fingers on the keys) or, most commonly, traditional sheet music.
Piano tab in terms of traditional sheet music is the written notation for a piece of music. It tells that musician what to play, for how long and in what sort of manner. Notes are dictated via a series of ovals, often stemmed, on a musical staff consisting of five lines and four spaces. Where the notes are located vertically on the staff indicates what pitch is to be played, and the note's physical appearance in the piano tab indicates its duration. Piano tab also includes other specifications like time signature (the song's meter), key signature (the key in which the song is played) and tempo (the speed at which the song is played).
Not all piano tab indicates the same things, however. Some piano tab, especially that found in jazz ensembles and popular music groups, is very limited, notating only the bare essentials of chord progression and tempo. In this case, piano tab is really just a quick reminder of what needs to be played; it certainly isn't relied on for the detailed nuance of the song. And some piano tab, often referred to as a score, is extremely detailed, including not only the piano parts, but the parts of every other instrument as well. This type of piano tab is intended mostly for conductors, though sometimes pianists use condensed versions as a guide to the overall melody.
___________________________
