Watch the Dominant 7th Chord online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners
7 chords have some dissonance in them. Dissonance creates tension, and tension wants to resolve. So one characteristic of 7 chords is that they lead your ear to another chord. As shown in the examples here, where the last chord is a 7 chord, it pulls back strongly to the first chord of the progression.