Five instead of seven
Penta means five. The pentatonic scale leaves out two notes of the regular scale — precisely the notes that can clash most easily.
Minor pentatonic uses the notes 1, ♭3, 4, 5 and ♭7. In A minor that's: A C D E G.
Major pentatonic
Major pentatonic uses 1, 2, 3, 5 and 6. In C major: C D E G A. Those are the same notes as A minor pentatonic; only the root is different.
Why guitarists use it so much
Because there are no 'dangerous' notes in it, almost anything you play sounds good over the chords of the key. That's why minor pentatonic is the basis of many rock, blues and pop solos.
Add the ♭5 (in A minor that's E♭) and you get the blues scale. That extra note is called the blue note.
Practice tip
On the fretboard map, select A minor and choose Pentatonic. Play the notes around fret 5 up and down and always finish on an orange root note.
Read also
Guitar Fretboard Notes
How the twelve notes are laid out across the strings and frets.
Major and minor
Why one key sounds cheerful and the other darker.
Soloing over the 12-bar blues
The most famous chord progression in the blues, and how to solo over it.
Pentatonic licks that always work
Four classic licks in A minor, with tab.