The thumb-and-forefinger-circle that means "OK!" to you means "Up yours!" or "You're an asshole!" in some cultures (especially when held upside down).
Never, ever hold out just your forefinger and pinkie raised to make "horns" (or the "I love you" gesture from sign language). Depending on how you hold your hand and where you are, you're either casting the Evil Eye on someone, warding against it (which insults the people around you, implying that they are casting one), or calling someone a cuckold.
In most cases, of course, your gestures won't mistakenly be offensive; they'll simply be misconstrued.
When holding up their fingers to count, most Europeans start with the thumb for "one." Holding up just your forefinger means "wait a sec" in most countries. You may be trying to order one beer; they'll think you aren't ready yet.
A southern European gesturing "come here" looks like they're shooing you away. People waving good-bye in Europe hold their hand out, palm facing up, and repeatedly slap all four fingers as a group toward themselves; to an American, that means "come here."
Confused? You should be. There are whole books on this subject. Gestures are an integral part of communicating in Europe, especially the south, but until you learn the European body language, it might be best just to keep your hands to yourself.
Body language - when is close too close?
Proximity is a relative thing. Be prepared that in northern Europe, especially England, people require a larger sphere of personal space than most in the south do and are rather adverse to physical contact. Dear old friends will rarely greet one another with anything more than a firm handshake and warm smile.
When you get to Sicily, though, total strangers will be throwing their arms around you and greeting you with a wet kiss on each cheek.
Of course, those are both stereotypes, but ones that often hold true.
The farther south you go, the more people touch. Manly, macho Mediterranean men will often link elbows and walk arm-in-arm with each another down the streets. Teenage boys, oozing hormones and unnecessary aftershave, will zoom up on their scooters and greet their pals with that double cheek peck so beloved of Hollywood types. They probably aren't gay, effeminate, or even metrosexual. (In fact, most Mediterranean men are about as heterosexual as is humanly possible; their blood is 90% testosterone.) That’s just the way their culture is. Men touch.
As always, be observant and let the locals be your guide as to how to act, when to be politely standoffish, when it's acceptable to sling an arm around someone's shoulders and plant a big, wet, frencher on them (note: only if you know them very well, or are very, very drunk).