Guitar chords, (chords played specifically on a guitar,) differ only from other types of chords by virtue of instrument; they're simply a series of three or more notes played together. These notes don't necessarily have to be played simultaneously, however.
Broken chords (also referred to as arpeggios) are three or more notes that aren't played at the same time but closely enough to still be heard as a group or whole. And even the three-note rule is open to the occasional exception; some guitar chords consist of only two notes, but they still function as chords because they work diatonically in the same way that a major or minor chord would.
Guitar chords might very well be the most important element of guitar playing; after all, they're the basis of what makes a song. Most people picking up a guitar for the first time figure out a few guitar chords before even going for their first lesson, and still more teach themselves guitar chords without any help from an instructor. Self-taught guitarists learn guitar chords in a number of ways. Some learn by listening to their favorite songs and slowly picking out the notes, a common yet often frustrating process. Others figure out guitar chords by learning to read guitar tab, a type of sheet music intended for fretted instruments that uses a graph-like chart to show where on the frets the fingers are placed. Both techniques are common among those learning guitar chords, though the number of self-taught guitarists who never learned to read tab is fairly high.
Just like any other instrument, the sheer number of possible guitar chords can often be overwhelming for a new guitarist. And even the frequently taught guitar chords are beginning to fall by the wayside, making room for a variety of guitar chords created by tuning the strings in almost innumerous ways. Though power chords (guitar chords using a base note, an octave note and the fifth) are still the most common type of guitar chords, new bands are increasingly experimenting with alternate tunings to create new sounds; alternative bands have been toying with this way of playing interesting guitar chords for decades.
So how many chords does a guitarist really need to know?
Most simple songs contain just 3 chords - called "primary chords". So even a stark beginner can learn 3 simple chords well enough to strum along and accompany himself as he sings. But after that, the sky is the limit - there are thousands of possible chords, so it is up to the individual guitarist as to how many he or she want to master.
(With Mollie Wells)