A release of gas via the mouth, the entryway of the gastrointestinal system, is a burp. Otherwise known as a belch, or for analogy purposes - a mouth fart. This gas generally comes from the air swallowed when chewing, or from carbonated beverages. Air causes those bubbles in the soda you drank, and it has to escape the stomach somehow - what better route than the way it came in!
What's in a burp? The air we breathe in contains mostly oxygen and nitrogen, so that's what we burp out. Carbonation is due to carbon dioxide gas, so a soda or beer burp contains carbon dioxide. Because the air is traveling up from the stomach where food is being digested, sometimes the burp contains a little more than gas, with the air pushing some of that not so tasty stomach juice up the esophagus into the mouth.
Slang term for this: vurp
What causes the burping noise? As the air passes through the muscle controlling the esophagus, the esophageal sphincter, the tissue vibrates. The more air being forced through, and the more strongly it is pushed, the louder the noise.
Gross fact: The world record for the longest burp was set in Italy in 2009 and lasted more than 1 minute 13 seconds.
In the picture: Astronauts have to drink soda from a special can that prevents them from swallowing air because the lack of gravity means that a burp contains more from the stomach than just air.
Read more about the gastrointestinal dilemma of space shuttle travelers at the Wikimedia page for the image.