Another important group of chords. Dominant 7th chords are great transitional chords.
Try to play chord G then G7 and then finish on C. Can you hear how it is resolving to the C chord? You can hear this kind of resolution in a lot of songs. Dominant 7th chords (or just simply 7th) are also commonly used in blues music. They will give you this nice tensed bluesy sound.
As practice, try firstly to play the major equivalent and then 7th chord. For example: play A then A7, or E then E7. You will hear the difference between those two. Can you hear the tension?