Total newbie.
Hi! Welcome to RPoL!
For the most part, it's what is known as PbP (Play-by-Post). Just like around the gaming table, the GM tells you something, and you respond. Only here, both of those things are done in writing. The GM writes a post and then all the players can post a reply. Whoever can post first simply does so, and the GM sorts out what all the characters are doing.
Role-playing is strongest with PbP, while combat is generally the weakest... because it can take several days to resolve a single combat.
The biggest thing to realize is that PbP tends to be rather slow, so you need patience. Also it's about writing.
There are games that use Skype, and there are Virtual Tabletops, but I don't know if there's anything like that done on this site. There is a General thread called Offsite Gaming. That can probably answer those questions.
Make sure you read over the FAQs up in the upper right corner. Even the Help board is useful.