I live in the United Kingdom, which is part of europe. Most of Doc No's writeup is right, but one or two corrections are in order.
In the UK, we drive on the Left.
On complex stoplighted junctions, you will sometimes have more than one set of lights. For example, there may be another set of lights on the other side of the junction that are easier to see. The light that applys to you is on your immediate Left.
Stop lights go Red (stop), Red+Orange (prepare to go), Green (go), Orange (Prepare to stop), Red (Stop).
Roundabouts go Clockwise (as you look down on them) in the UK (We also drive on the Left, so I guess it makes sense.)
The Fast lane / passing lane distinction doesn't always apply. It's best to look at what other people are doing.
Check up on the speed limits for the country you are visiting, before you get there. They are different in different countries.
British cars typically have large MPH, small KPH. Many street signs are in MPH too. I'd expect hire cars in most countries to have both mph and kph, possibly varying large and small.
Gas = petrol. People in the UK call it petrol, not gas. Don't ask for directions to a gas station; one American friend of mine ended up at a compressed gas supplier. You don't want that to happen. So ask for petrol.
Petrol is expensive. In the UK, it's about 77 - 80 pence per liter, but it varies depending on where you get it. That's $1.09 to $1.14 per liter (Assuming £1 = $1.4235), but this guide will not be valid forever, due to changes in the prive of petrol, conversion rate changes, etc.
If you prefer imperial, petrol is around $4.96 - $5.19 per gallon.
Be careful. Keep hold of your passport. Make sure you're on the left side of the road. It's only driving; how hard can it be?