HI,
Geocoin is a type of trackable. People usually collect them or let them travel the world. Just another fun thing in geocaching world.
Trackables are objects (coins, tags, etc.) with a unique number, engraved or attached on the object in some way.
If you buy/receive a trackable, you register it with the unique number and activation code on the website, and it goes to your inventory & is listed as your owned trackables (your profile>Trackables>right column). Then you can either keep this trackable with yourself (take it to events to show off :D) or release it to travel. If you choose to release, you simply put it in one of found geocaches (you'll have an option to drop off trackable when writing a log) or simply give it to somebody else (good option if you know a cacher who is about to travel to a different state/country).
If you find a geocoin in a cache, you can enter the code on the website and "discover" it - you'll see its description, goals, stats - and it will be listed under your discovered trackables in your profile. You're not supposed to take it and keep for a long periods of time, trackables should move from cache to cache.
The difference between geocoin and travel bug is mostly just appearance. Geocoins are more often used for building collections than releasing them to travel around the world.
Hope that answers your question.