[kabar]

The medals are separate from the ribbons. The medals themselves consist of fabric with a medal hanging from it. The medal is about the size of a silver dollar or a bit larger. The ribbons (bars) are much smaller and are used separately. You don't wear both at the same time.

[raynearhood]

You don't wear both, but they are completely interchangeable. Either can be worn on Class A uniforms. Traditionally, the full medals are worn on full dress or formal uniforms (Navy whites, Army blues...) and the ribbons are worn on the casual dress uniforms (Army greens...) or Class B uniforms (the khakis and greens without the jackets).
However the ribbons represent the medals. Same placement and respect is given to both.

Note: Not all ribbons represent medals. Some ribbons are called service ribbons and represent accompishment. In the Army we have the NCO Professional Development Ribbon, the Army Service Ribbon, the Overseas Service Ribbon, and a couple more.

[Empireoftheatom48]

That's right! usually you would wear your ribbons on your class A uniform. On a full dress uniform (Blues) in the Army, you would wear the medals full size some times in miniature depending on the type of dress uniform. Also you could wear your ribbons on your Khakis. We did not wear them on our work uniforms. However, you see Kerry and scum wearing them on their shoddy work uniforms. I'm not sure if you could do that in the Navy at that time.