Pentatonic Scale Theory Things   William Wieland
Strictly speaking, a pentatonic scale is simply a 5 tone scale.
However, a pentatonic scale usually refers to a major pentatonic scale;

1 – 2 – 3 — 5 – 6 — (8) of a major scale or do – re – mi — sol – la — (do)
To create a pentatonic scale, simply remove the 4th and 7th notes of a Major Scale.
C major scale
  C pentatonic scale
(C major pentatonic scale)
E flat major scale
  E flat pentatonic scale
A major scale
  A pentatonic scale
Bobby McFerrin Demonstrates the Power of the Pentatonic Scale
The black keys on the piano are an F sharp (or G flat) major pentatonic scale.

Piano Pages:  Pentatonic Playing by Ear   Pentatonic Improvising