Diablo 3 features a four difficulty system, whereas its predecessor only had 3 difficulties. In addition to the standard difficulty sets of normal, nightmare, and hell, Diablo 3 adds in the Inferno difficulty once you beat Hell. Inferno is much more challenging than the other difficulties. It is meant to be the ultimate test of your Diablo 3 mettle. It is estimated that only a small percentage of Diablo 3 players will ever beat the final act of the Inferno difficulty setting.
Inferno is a huge step up from the Hell difficulty for several reasons. Your resistances really matter for the first time once you reach Inferno. Elemental and other non-physical damage types will be coming at you from a variety of sources. Just having high armor or vitality statistics will not be enough to make it through inferno. Without massive amounts of resistances you will be spending the vast majority of your time waiting to revive.
Another reason Inferno is so difficult is because the unique and champion mobs have 4 properties on them. It is not uncommon to find a group of champion mobs with a property combination like jailer, vortex, arcane, desecrator. These big combinations can cause huge problems for players, sometimes forcing them to reload the game and hope for a different combination.
The game shifts rapidly into more of a team dynamic when you reach Inferno. It is simply too hard to push forward alone with all of the increased difficulty. With the aid of your fellow players, you can work together to combat the greatest challenges Diablo 3 has to offer in its new Inferno Difficulty setting.