How to Play the Pentatonic Scale on Guitar - dummies
The first scale you learn is the A minor pentatonic. A minor pentatonic can be used as a lead scale over chord progressions in A minor, C major, and A blues ("blues" can imply a specific, six-note scale, as well as a chord progression). It also works pretty well over A major and C blues. Not bad for a scale that's two notes shy of a major scale.