I'm talking about the Eminor Pentatonic scale (Fig. 1) and the E Blues Scale (Fig. 2)
Since guitar players love to play in the key of E, (big sounding open-string chords like E and A) the Eminor pentatonic and E Blues really fit the bill.
Pentatonic scales are useful when improvising and composing because they refine the sound of a scale by removing the notes that cause 1/2 step intervals. These notes are the ones that tend to cause clashes or dissonance. In the case of minor pentatonic, the 2nd and 6th note of the minor scale have been removed leaving the 5 note scale found above (Fig. 1).
The blues scale expands on the sound of the minor pentatonic by adding the "blue" note. The "blue" note is an approximation of the sounds made by early singers of field hollers and work songs. These notes where generally slid into or "bent"vocally and gave the vocalist a more plaintive or yearning sound. Guitarists will often bend into and out of these notes to simulate the sound of the human voice.
Try out each scale and compare the sound of the minor pentatonic and the Blues scale.