What is G rank? Or even high rank.. :X Do I have a rank? I'm doing the 5 star village quests
Think of them like difficulties in a Diablo game. Low is normal, high is nightmare, G is hell. Except it's not quite the same and each difficulty expands on the content available.
The original release of MH4 (and other mainline MH games) included low rank and high rank. Once you clear low rank you get into high rank which has stronger versions of the older monsters, as well as many new monsters and subspecies. Some monsters gain new moves, most are faster and hit harder. The materials you get in high rank are used for all new armor sets, weapons, and upgrades for low rank weapons.
G rank was added to this game, which is a standalone expansion of MH4. It's another difficulty level above high rank, with even more new monsters and subspecies. Monsters gain more new moves, and of course the equipment is much more powerful and varied.
The Caravan story progresses through low rank, then you beat the 'final boss' and the vanilla MH4 singleplayer content ends. From there you move on to the high rank content which has its own story and end boss. The story stuff stops there but you unlock some more G level quests by doing the online 'guildhall' missions.
Guildhall stuff is separate and is where your hunter rank (different from low, high and G ranks) comes into play. You raise that rank by doing guildhall quests and it determines which quests you can do. It's roughly equivalent to the caravan star ratings, at least until G rank where it serves a different purpose. Worry about that later though.