All joking aside, tipping in the US (at least for servers) is a huge part of their income and they actually get taxed on the tips as well. I used to be a waiter back in college and know this first hand. Minimum wage in the US is around $7.50/hr however if you are a server the employer can pay you as little as $2.50/hr (it varies by state) because they count the tips they receive as part of the minimum wage. As long as your tips + the crappy wage they pay you ends up over $7.50 it is considered OK. Therefore, it is customary to tip about 15-20% here in the US. All credit card tips are automatically taxed (because they are in the computer system) and most employers count at least 10% of a servers's sales as the minimum for being taxed. So if you sold $1000 worth of food in a night, they would assume you made at least $1000 (even though you probably grossed about $150 @ 15%) and tax you on it. You would also get a measly little check at the end of the week because they would calculate out the hours you worked, times the "server minimum wage" and then take the taxes out of it calculated on both your sales and what you claim. Lots of times my checks would be like $30-$50 for around 30+ hours of work due to the taxes.
Depending on where you serve food here, it could be quite a lot of money depending on the price of food you serve (say at a fancy seafood lobster restaurant) or it could be crap (say at a waffle house/breakfast shack). Also, servers here in the US often times have to split their earned tips between the bartenders who make alcoholic drinks for their tables, the busboys who clear the tables, and the runners who bring food out from the kitchen. Not to mention that they work every holiday, and usually Fri/Sat nights (where the money is). It's safe to say that I don't miss that profession any more..but it could be great money for a kid going to college.
Other tips here in the US are usually for taxi drivers, bellhops, tour guides, babysitters, dog walkers, valet parkers, and even sometimes those who give you food in a coffee/donut shop. These are optional and nowhere near the customary 15% for servers. Depends on the amount of the service but it could be an extra $1 to an extra $5.
Totally different than the EU or other places I travel to around the world!