What are chords?

A chord is formed when multiple pitches (notes on a musical instrument) are placed together on a staff (a sheet of music). Chords come in a lot of different varieties, including (but not limited to):

Major chords, minor chords, diminished chords, augmented chords, seventh chords, and Neapolitan chords.

To develop a chord, one simply takes two or more notes from the staff and (in the case of piano or string instruments) plays them simultaneously.

For instance: C-E-G is a C Major triad (triad is just a fancy term for a chord with three notes). F-A-C is an F Major triad.

When you listen to music, the harmony parts behind the leading melody are generally chords. Rock groups like "Queen" are notorious for having a lot of harmonized parts in their music.

Every song from every genre has a chord progression - meaning one chord follows the next in a pattern. Rock is generally I-IV-V-I (Roman Numerals denote the type of chord used) used over and over.