An arpeggio is a technique where the notes of a chord are played in order one after the other as quickly as possible, starting on the beat from the bottom note.
Rather than a strum, where all notes are played more or less simultaneously, the notes of an arpeggio are 'spread' out and played in quick succession.
Chords can be arranged with all its notes in ascending or descending order, so an arpeggio can be played upwards or downwards.
On the guitar, this can simply mean to 'rake' the pick or finger across all strings of the chord in one movement, but with enough time between the notes to let each individual note sound before the next is struck. Here are some examples:
However if we want to arpeggiate bar chords, we have to re-finger the chord as the strings are picked to accommodate each note in order (1st,3rd,5th etc). Try these examples:
Arpeggios are movable to any key, and can be performed on any chord. I have shown here the more common arpeggios, however there are many others. Try to work out arpeggios for minor 7, minor 6, dominant 7 and major 9 chords.