polish is like moisturizer for the paint.
wax is for protection, like bees-wax on fine wood
clay is similar to the polishing method used in tha fer east on pottery, it smooths the surface, rids it of imperfection.

the order of use on your car would be; wash.clay,polish,wax

if you rub your fingers across a freshly washed paint finish it should feel smooth like glass. If the finish is not glass smooth a liquid paint CLEANER like that sold by Meguiars or any other reputable car care manufacturer would be a way to get the paint smooth. Another method if cleaner didn't work is to use clay with a lubricant.

You always want to use the least agressive method first, If wash and polish and wax don't give you the desired effect you can move onto cleaner or clay.

Be extra careful with black paint it will scratch easily, so always use the least agressive methods.

If this is your first detailing of your car I would not clay. I would wash and wax. Then depending on the paint finish I would consider using cleaner, polish and wax at the next detailing. If paint then is not smooth I would then consider claying.

Use the search forum to learn about claying. Also do a search under '303 Protectant' for protecting plastics and vinyl