A 'pentatonic' scale is a 5 note scale. "The pentatonic scale" is a common reference to the minor pentatonic scale, a sound which has been common in mainstream popular music for most of the last century, especially classic rock such as Led Zeppelin, Pink Floyd, Black Sabbath, or anything in the blues rock category of music.

5 note scales are special. They can be seen as more simple versions of their 7 note scale counter parts from the major scale modes. They represent ultra stability in the key, making them extremely safe notes to use in melodies and solos. The above minor pentatonic scale is based off of the natural minor scale, but excludes the 2nd and 6th scale degrees.