Watch the Dominant 7th Chord online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners

Dominant 7 chords - often abbreviated dom7, 7th, or simply 7 - are very common. You're bound to come across them as you pick up sheet music in most any style of music, from rock to alternative to folk, funk and blues.
Look at the chord grids in the video and in the accompanying pdf to see how the major chords here are changed to 7 chords by changing just one note.