What's the difference between a dominant 7th, a diminished 7th and a major 7th chord?

The dominant 7th is commonly used in classical and jazz repertoire to lead back to the tonic, often as part of a perfect cadence. Referring to the dominant 7th chord itself, the 3rd rises by a semitone and the 7th falls by a semitone when we move from dominant 7th to tonic. This is an example of semitonal pull. A diminished 7th is a chord built upon consecutive minor 3rds, and tends to be used to create tension in a phrase. Due to the fact that there are only three different diminished 7th chords possible, they are often used as pivot chords to modulate keys. The diminished triad is found in the harmonic minor scale, thus - in classical repertoire - the diminished 7th chord is more often found in pieces of a minor key.The major 7th is a chord found very commonly in jazz, less so in classical. It is used commonly as a decoration of a major chord. The major 7th chord is, notewise, the same as the dominant 7th except the "7th" is a semitone higher.