It depends on the place. Many (non-fast food) sit-down restaurants will have 1 waiter/waitress (the PC term is "server" now) that generally takes your order and brings your food and the tip goes to that person as they are assigned to that table. However, a lot of chain/every day restaurants have changed and there's a pool of servers so you may not get the same person checking up on you, not to mention the busboy and/or person who refills drinks. Some of these latter restaurants will have a pool and split the tips up based on certain rules. For large parties, the tip can be factored in to the bill automatically, whether you like it or not (usually 15-18%). Average tipping rates are around 15% of the pre-tax bill. Really good service usually warrants a bigger tip, 20% or more.

Then there's your smaller restaurant, cafe, coffee shop, and some fast-food restaurants that have a tip jar so you can leave tips if you like, whether it's just the change received or an extra dollar or two. If you're a regular, whether or not you tip is usually noticed. The jar is split up based on whatever the shop decides.

In bars or pubs, a usual tip is $1 per drink. More if you think it's worth it, are at some super fancy place, or you simply like the bartender.

What is a bit dumb is when you have a place like a self-serve yogurt joint that has a tip jar. They do nothing warranting personal service other than ringing you up because it's entirely self-service, so it seems a bit like begging.

Some places that don't have tipping (like a McDonald's type of fast food place) will occasionally have a customer who tips. This is rare and usually a surprise.

What's funny is that I learned many Japanese-run sushi restaurants prefer American customers because Americans tip. Japanese do not tip in their country, and either leave none or a very small tip. I assume this is true with some other types of restaurants and attitudes towards tippers.