Like the description said, this is the biggest Monster Hunter game up to date. Monster Hunter Freedom Unite, also known as Monster Hunter Portable 2nd G, is an expansion of Monster Hunter Freedom 2. Several new monsters and weapons are added, the Hunter Rank have increased from 6 to 9, that means there are more, a lot more missions for you to do, either by yourself, with your felyne friend, or with other people.
If you're new to the series, this game require a bit of skills and patience to get good at, since the monster can take you out in a few hits. That means you can't just jump straight in the game and expect to take out a monster by randomly slashing it. The game isn't that fun if you play by yourself, but it can become addicted if you have someone else to play with. I never liked the series until a mate of mine make me play with him. I ended up with over 200 hours of play time in less than a month. It is truly the best multiplayer PSP game ever made.
If you have a PS3, you can play this game online with other players via Ad Hoc Party, a neat add-on that you can download from the PSN store; it will let you play supported PSP games online. With over 2 millions copies sold in Japan alone, there are always players available online so you won't have to worry about finding someone to play with online.