Hi mikey!
An ATV is an All-Terrain-Vehicle, or 4x4. It was the first time I had ever been on one and it was great! They're very easy to ride, you just have to get used to them (and watch the corners when you're turning - but you'd have to be careful with just about anything you drive.)
Apparently, they are becoming quite popular in Greece and the locals call them "gourounes" which means "pigs"
The only problem we encountered was not knowing where to put our belongings - there is no storage bin on them.
Let us know how your trip goes!

delfini