Primer is used to prime the part before the the real coat of color you want to add. Primer will give the part a better look for painting as you will not have to use a lot of coats to cover a dark color.
Base is the same.
Top coats are to keep the paint from chipping and wearing off. A Flat top coat will give a flat look. A Gloss spray will give it a gloss look.
When spraying stay about 12in from the part and mist it moving your can side to side. Do this to not have the paint build up in just one spot.