T.I.P. stands for To Insure Promptness, and was originally used like we now use "to grease" or "to slip (someone a twenty)". These days tipping has taken on a new role in the way we receive service and there are situations which require different approaches. These are the basics of tipping after a meal.
But first, when DON'T we tip? We are not expected to tip, unless given something for free, when the server is paid a wage. For instance, most behind the counter workers like Starbucks' baristas or your local deli guy are paid a wage. Your kindness may not even be felt some baristas, Starbucks recently ran into trouble by giving a share of tips earned to manager. To read further on this check out BBC. Still, it is nice to acknowledge people's work for you with money.
If you have never worked in the restaurant industry you may not know that waiters/waitress' work almost exclusively for tips. This means that they are paid only as much as you give them for the period of time you are sitting at their table. In America, it has become mandatory to leave gratuity after your meal, because restaurants do not pay a wage for service. It is standard to pay 15-20% of the meal you ordered. Any less could only be because of atrocious and audacious service, and not necessarily linked to the food itself.
Remember: If you can't afford the tip, do not eat out.