The essence of RPG gaming boils down to some very basic elements.
Rules. A significant amount of time is spent looking up, or making up rules. What happens when a person in plate mail jumps in a lake? How long can he stay under water? Etc.
Background Information. Some time relaying background information occurs, except in the rarest of circumstances. Sometimes the players bring this information to the table ("Oh, this is Greyhawk, the Inn we are looking for should be this way...") or perhaps from GM handouts, or in story form. "As you all know, (stuff you didn't know)"...
Encounters. The true heart of what the Game Master does lies in the encounters. Encounters ususally consist of places, some rules related to the encounter area (d6 falling damage, plus d6 spikes for d4 each) and creatures (either friendly or hostile). The players interact in the encounter, either gaining experience, treasure, or background information, or moving the story along in some way. Well, sometimes, players will ignore and destroy story lines, but that is another story...
Encounter Areas. Encounter areas can be quite small (waking up in a coffin) or quite large (a large bustling metropolis). They can be very detailed, or only vaguely defined. They can be stationary (The Dragon Inn) or mobile (A Pirate Ship, or Dr Who's TARDIS).
Campaign. The campaign world is really just a backdrop, a setting in which encounter areas are linked, and a source of background information. This can be as tenuous as nothing (no back story linking encounters at all), or
in-depth, or even Exhaustive (trying to use all published campaign material at the same time).