Look up 'tipping' on wikipedia for everything you could possibly want to know. Here is a section I pulled regarding restaurant tipping in the USA:
At a restaurant
Tipping is customary in restaurants having traditional table service. As tipped employers generally qualify for a lower statutory minimum wage from the employer, tips constitute the majority of the income for tipped workers such as wait staff. While opinions vary, Fodor's suggests that tipping wait staff 15% of the bill (before tax) is considered standard, or up to 20% at more expensive establishments.[33]
Some restaurants include an automatic service charge (not to be confused with a discretionary tip or gratuity) on the bill for parties of six or more, while a rare few include an automatic charge for all bills.[10] Reputable restaurants and servers will advise customers of this mandatory charge before they order.
Tipping on wine with a meal requires some discretion/judgement. Tipping etiquette websites[34][35] suggest a tip of 15% on the meal before tax, and 5-10% on the wine (especially if the total wine bill is near or exceeds the cost of the meal).
Tipping for traditional table service restaurants offering take-out is not necessary, although a small tip (e.g., 5%-10%) is sometimes suggested for curbside service.
Tipping at buffet-style restaurants is not necessary, although some may think it is appropriate to tip a small amount (e.g., $1 per person) if a server refills their drinks or 10% if there is some table service in addition to the buffet.
Some coffee shops or establishments without table service leave a countertop tip jar (a.k.a. guilt cans) by the cash register, but tipping is not expected as it would be for table service.[32]
Servers usually "tip out" portions of their tip receipts to support staff like bussers and bartenders. Bartenders usually are also tipped by the patrons ordering drinks at the bar.[36]