It's burping, belching, or more technically, eructation. "Eructation" was borrowed from Latin in the 15th century; the verb eruct, meaning "to belch". The suffix "ation", also from Latin, is the action or process of doing something.

Eructation refers to a process where gas is expelled out of the stomach, up into the esophagus (the food tube), into the mouth, and then out of the body for everyone's sensory enjoyment.

Think of your stomach as a balloon. It also may reach a point where the gas inside of it will cause its walls to expand too much. The stomach doesn't like this and so it burps away the excess gas in order to prevent discomfort.

There are plenty of causes for belching and I'm sure you can relate to some of these. Have you ever had a lot of soda? Well, the soda is carbonated. Those little bubbles in the soda that fizz are actually gas. So when you drink it, not only do you get a sugar rush, but you also pump up your stomach with gas. This is gas the stomach doesn't want or need and it squeezes it out of your body through belching.

If you have ever eaten something way too fast, you may also belch quite a bit. This is because eating or drinking too quickly may cause you to swallow a lot of air and that air has to be burped out.

Some cultures regard burping as acceptable in certain situations, for example in South Asia it signals the host that the guest has enjoyed the food and is full.

More Info: en.wikipedia.org