In America, this usually means, "Great job!" In Greece, Latin America, the Middle East, Russia, and elsewhere, it means "Up yours!" If you really want to be offensive, you can thrust your thumb upwards to emphasize the offensive message, but odds are you really just want to avoid it.
You might use this gesture to delicately wave off someone's offer of a drink refill, or to tell someone to stop when they're parallel parking. But in Greece, Africa, and Pakistan, this gesture represents you rubbing someone's face in excrement. That seems like a really specific thing to have a hand gesture for, but who are we to judge a good insult? This gesture has also taken on some aggressive sexual connotations in recent years.
Making a circle with your thumb and pointer finger usually means "OK!" in the U.S. In some Middle Eastern countries, it represents the evil eye. A more widespread meaning, though, is "you're an asshole." It's taken this way in Greece, Spain, and Brazil, and there's an extra layer of homophobic subtext to the gesture in Turkey. Nixon once famously made this his first impression to an entire country. Don't do the same.
Originally "v" stood for "victory." Nowadays, it's used as the "peace sign." You see it often when people are trying to invoke hippies, and it's such an iconic part of taking photos in Japanese culture that Time Magazine did a write-up on where it came from. But in Australia and the U.K., this sign (with palm facing inward) has had the same meaning as the middle finger since at least the year 1330.
This gesture doesn't mean anything in particular in the U.S., which is just as well, because it has riot-causing potential elsewhere. If you group the fingers of your right hand together, while pointing to them with your left, you're telling someone that they "have five fathers"—in short, you're calling them a bastard. This one is particularly egregious in most Arab countries and in the Caribbean. You're not as likely to do this one on accident, but given the intensity with which it's received, it's only fair that you know about it from the outset.
To an American, crossing your fingers is a slightly cheesy way to invoke good luck. No one really does it outside of female leads in tone-deaf romantic comedies, but the meaning is still understood. In Vietnam, however, the gesture is believed to have an...explicitly feminine appearance. Consider it a female analogue to flipping someone off.
Hands Below the Table
It's drilled into American children not to rest their arms on the table when they're eating. But in France, the polite thing is to keep your hands on the table, palm down, on either side of your plate. When your hands disappear below the table, your host will be wondering what exactly it is you're doing with them.
Sign of the Horns
In America, this gesture generally means that you take your Texas roots a little too seriously. Depending on thumb placement, you might also identify yourself as a metalhead. In Brazil and Venezuela, this gesture is a wish for good luck. For people in Italy and Spain however, this means their spouse is cheating on them. Furthermore, in Portugal, Colombia, and Brazil, this gesture means "your wife is unfaithful," often with the corollary, "and you're too stupid to realize it."
Using Your Left Hand
This one's rough. There are still places in the U.S. where people are given a hard time for using their left hand, but in large swaths of Africa, Asia, and the Middle East, the left hand is still mired traditionally with connotations of personal hygiene. To give a gift with the hand that's traditionally used for toilet paper is a big no-no, as is using it to eat. Also, it's not recommended to go for the handshake.
This gesture, a closed fist with your thumb between the first two fingers, is usually only seen when your uncle or grandmother has "got your nose." Unfortunately, it's heinously offensive in large portions of the world. Turkey, Russia, and Indonesia will read it as a big "screw you!" In some countries, especially Italian-speaking ones, the gesture is seen as being evocatively feminine, in much the same sense as crossed fingers in Vietnam.
Anyone who's been to middle school in the United States knows about the obnoxious "loser" L on the forehead. But in China, there's no ill will behind this gesture--it's simply what they use to indicated the number eight, which is particularly lucky in their culture.
If you want to look fancy (or at least pretend to look fancy) when drinking tea, you might pop your pinky up as a little flair of refinement. But in China, this gesture is a real downer--it's basically the equivalent to an American thumbs down. If you see this gesture there, the person you're speaking to is clearly not happy about something.
"Come Here" Finger
When you beckon someone with your index finger in America, it could mean a few things--you might use it to look seductive to a partner, or a parent might use it on a misbehaving child. But in the Philippines, this gesture is an absolute no-go. This gesture is only used to call dogs, and if you try it on a person, you might end up getting arrested!
The "Italian" Gesture
In the United States, if we want to seem "Italian", we bunch up our fingers and wave our hand around. Strangely enough, this is actually a gesture in Italy as well--but they don't use it to assert their Italianness. Instead, they use it when something is unclear and needs further explanation.
Smelly Hand Wave
In America, when we see someone waving their hand in front of their face, we know they've smelled something terrible. But in Japan, this sign can be used to say no or indicate that something is impossible.
Money talks in the United States--so much so that we came up with a gesture for it. But if you make this sign in South Korea, they won't think you're money obsessed. Instead, they'll think you're romance obsessed, because this gesture means "love."
Animal Horns on Head
This gesture isn't really a big thing in the United States, but we might use it to pantomime a horned animal or just to be silly. However, in Japan, it's not such a lighthearted symbol. Rather, this gesture represents demon horns and indicates that someone is incredibly unhappy.
Snapping fingers mean a lot of different things across the globe. In the United States, we do it when we're trying to remember something, but in Latin America, it's a request for someone to hurry up. And in other cultures, it's seen as just plain rude.
In the United States, if we want to mime a phonecall, we might take our thumb and pinky and place our hand up to our ear like a reciever. However, if you were in Hawaii, this gesture might mean relax, while in Germany it means you're trying to order a drink.
Pointing at Someone
For a long time, pointing at someone could be considered a semi-aggressive gesture in the USA, but over time, it's also come to be a humorous sign of approval. However, in countries across the globe this gesture is considered especially offensive. For example, in some African countries, it's only appropriate to point at objects, never people.
Handshakes are a tricky thing in the United States. A firm shake is a sign of respect, but if you squeeze too hard, you're likely to be considered aggressive or arrogant. In Japan, the bar is much lower for what counts as "too hard." What many Americans would consider a polite, firm handshake would be seen as overkill there.
Waving goodbye is second nature to Americans. It's an uncomplicated gesture that's unlikely to offend anyone. However, in parts of Europe and South America this gesture actually means "no" instead of goodbye.
For Americans, nodding along to someone talking means that you agree with what they are saying, or it can simply be a way to convey that you're actively listening to them. However, in Bulgaria and Greece this gesture means the opposite--you're disagreeing with what you hear.
In the USA, if we want to insult someone, we might flip them the bird. The middle finger in our culture is seen as one of the most disrespectful gestures you can use. However, don't get offended if you find yourself in a sea of middle fingers in China--it's the digit they normally use to point with and conveys no ill will.
Sometimes in America crossed arms can be a mild sign of rudeness or anger, but lots of people also just naturally rest their arms in this way. However, it's best to avoid this gesture entirely in Finland--it can mean that you're looking for trouble or looking to start a fight.
If you shake your fists at someone in the United States, expect a fight. It's an aggressive gesture (when it's not being used playfully). But if you find yourself in Austria, give your fists a wave! It's a way that they use to wish people good luck.
Finger Across the Throat
If you want someone in the USA to be afraid of you, move your index finger across your throat--it's a visual way of saying, "you're dead." While the gesture in Japan is not a good one, it's not as drastic as in America. Here, it means, "you're fired."
When Americans want to pray, they sometimes place their palms together near the chest. However, in Nepal this is a common and polite way of greeting someone or saying goodbye.
Pointing at Your Nose
In America, if we want to identify ourself, we might place a finger on our chest. However, in Japan it's a little different--they self-identify by placing a finger on their nose.
We might poke someone's cheek as a silly sign of affection here in the United States, but in Italy, poking your own cheek means that you've eaten something delicious.