In the US waiter/es make about $3-$5 an hour, so they depend on tips. I was a waitress in college. If not for tips, I couldn't afford to live.
In this country (England) I never pay a tip by card because it is less likely that the waiting staff will ever receive the money that you want to give to them.
We leave cash tips on the table in this country for the waiting staff to collect.
Besides the tip 'yes or no' question, I also find it interesting that different countries have different ways of giving tips:
In Germany, we usually give tips directly to the waiter when paying. They'll come with the bill to your table and say something like "11.50€", I'll give them a twenty euro bill and say "fifteen euros" which means that they'll give me back five euros and the difference between 11.50 and 15 € is tip. I tend to give more tip than others because I'm not good at calculating the perfect amount of 10% at the table. In some restaurants, you pay at the till, in which case I usually don't give tip.
In other countries you leave the tip on the table, whether just a few coins or a few bills.