I have been trying out the Grunfeld lately and I don't understand some of the moves. Please help, any comments would be great. (Also I have seen 8. h3,

Obviously the rook isn't going to be pinned on a1. You probably read that white's pawns on d4 or c3 are pinned to the rook, beause of the bishop on g7. The rook move to b1 allows white to play d5 and c4 (not straight away, but at some point).

Rb1 also pressures the b7 pawn. This keeps the c8 bishop from going to g4 pinning the f3 knight. Black is trying to break down white's center. He already has three pieces aimed at d4 (g7 bishop, c5 pawn, and d8 queen). He can bring another piece there with Nc6, and attack one of the defenders of d4 with Bg4. Rb1 slows down the Bg4 idea, while at the same time, claiming a half-open file, as you noticed.

...d5: the idea is for black to immediately strike against white's center, in a way. In the exchange variation, white gets a strong pawn center, but black gets a queenside majority and good counterplay.

cxd5: forcing black to recapture with the knight, which will then be chased away with e4.

...Nxc3: black captures the knight to force bxc3, after which black has a queenside majority which can often turn into a powerful asset.

...c5: black strikes at white's center. If dxc5, Bxc3+ wins the exchange (Bd2 Bxa1 Qxa1).  After an eventual ...cxd4 cxd4, black's two to one majority on the queenside will be more easily utilized.

Rb1: white is simply moving the rook out of the line of the bishop on g7, making the idea above carry less force. The rook is also attacking the b7 pawn, making it harder for black to develop the light-squared bishop, as Loomis mentions.

Black will generally continue with Nc6 and O-O.

The idea is that if Nc6 than d5

Notice that 8. ... 0-0 9. Be2 Nc6 10. d5 then 10. ... Bxc3+ is a mistake. because white has

11. Bd2 Bxd2+ (forced because both the c3 bishop and c6 knight are hanging)

12. Qxd2 and now the dark squares around the black king are really weak. One simple plan for white is to just play h4 and h5.

9. ... Nc6 10. d5 Ne5 can be played by black but there are alternatives at move 9. For example:

9. ... cxd4 10. cxd4 Qa5+ and now 11. Bd2 Qxa2 and black snatched a pawn (white has compensation, the game is equal and difficult for both sides). Or 11. Qd2 Qxd2+ 12. Bxd2 and we're heading to an equal endgame.