It is a Role Playing Game, yes. Though there are many forms RPGs take. Examples of ones would be:
Bauldurs Gate, Diablo, Neverwinter Nights, Knights of the Old Republic.
It's a first person game, but that's about where the similarities end with games like Half-Life etc.
Really basic rundown, though not the best explanation.
You start off chosing what kind of person you are, race, what kind of skills you will specialise in. So for example, you specialise in being better at armed combat with a sword and shield, or maybe using bow and arrow. Or you might want to be better at magic and all that stuff.
You've got experience points and gaining skills. The more you use your sword, the better you get with your sword for example.
It's an adventure game where it's very, very open, you wander around the place, finding people, and they'll give you little quests to do, in return for money and other types of items you pick up on your travels. The vast majority of quests you do will be off from the main story. The whole game does have a main quest, but you can take your time on that quest and instead roam the lands gaining skills etc.
Have a read of some reviews and videos of ingame stuff, you'll get a bit of an understanding of what it's like.