DO NOT listen to other reviewers who rated this game low. This is a review by someone who doesn't suck at video games.
In SSX you basically have three game modes: Trick it, Race it, and Deadly Descent. Trick It has you competing against other riders to get the highest score by doing tricks. It is my favorite game mode because it is just such a blast racking up huge combos and leveling out the instant before you hit the ground to get a smooth landing. Race It has you competing against other riders to get down the mountain the fastest. Race It can be a bit more challenging than Trick It, however, because you need to keep doing tricks to maintain your boost to get down the mountain the fastest. The last game mode is Deadly Descent. This mode has you simply trying to survive the perils of the mountain. You have certain "gear" that are specific for certain Deadly Descents that are a requirement to attempt the descent. These gear help you handle each mountain's specific danger. This mode would probably have to be my second favorite because it is really fun just trying to survive the treacherous terrain.
Multiplayer could hardly be called multiplayer because there is no set start and finish against other racers as far as I can see. You basically pick a peak and there are thousands of other riders trying to get the highest trick score/best race time on them. When you are going down the mountain you will see other riders' times/scores and you attempt to beat these scores. But so far, I have not seen an option to just join a match with other riders and compete against them in real time, starting the race at the same time just like a single event match. I hope this option is in there and that I just haven't found it yet.
Overall SSX is a game that is just meant to play and have tons of fun pulling off crazy combos and racing down mountains at insane speeds, all the while having a blast because you know there is no way any of this could happen in real life. Somewhere, Sir Isaac Newton is turning over in his grave. What am I doing still on this computer? I've got some laws of physics to break.