A
chord consisting, in its most basic form, of the
root,
third,
fifth, and
seventh tones in a
scale.
The seventh is almost always flatted when seventh chords are used in popular music. Thus, a C7 chord would be C - E - G - Bb.
When playing a seventh chord from a fake book or lead sheet on a keyboard instrument, you generally leave the root tone out of the right hand part, replacing it with the seventh. Then play the root tone with the left hand.
When used excessively (especially uninverted), seventh chords can make your composition sound like 70s porno music or terribly, terribly hackneyed (for instance, try leading from a G7 to a C chord, and tell me if it could ever sound exciting). However, used judiciously, they can be truly graceful.