What's Monster Hunter about, you said ? Well, let me tell you.
It's a game in which you must hunt monsters that are way taller/bigger than your own character, to complete quests, upgrade your weapons and/or your armour, and fight even more monsters, except that there'll be more monsters, and you may have to fight them in different conditions (in the desert, reducing you health, or in a frozen place that will drain your stamina gauge faster, but these cons can be avoided
). And generally, at the end, you face a powerful monster.
You have more quests in the G/U versions of the games than in the regular one, though Capcom decided to add free DLC quests lately. On Portable 3rd (PSP), they even added extra weapons/armour that deal a lot of damage !
On home consoles and on the 3DS in the future, you can play online with anybody or your friends, and make the hunt for monsters even more funnier. Of course, you'll face "hardcore" variations of the monsters you have faced offline, but, the items they drop are worth it
.
My description might make the game sound redundant, but, believe me, it's funnier than what you might be expecting. And I'm not talking about the graphics.
---> Small thing: Don't go for MH3U if you have a 3DS, keep your money for MH4U, because this game will feature more monsters than all the other versions (88, to be exact, while MH3U has 73 and MH4 [Japan-Only] has 72 monsters.)
I'll stop here, because there's too many things to say about the game because of how good it is. Get one and play it for yourself, and you'll see.