Background info:
Path of Exile is a free to play online game developed by the New Zealand based company Grinding Gear Games, which will utilize micro-transactions on release. Its an Action Role Playing Game quite similar to Diablo 2 but it has modernized graphics that immerse the player in a gritty and realistic looking world. The whole game is instanced, meaning that every time you enter an area, it will be different allowing for lots of replayability.
Recent screenshot (oct 2012):

Game mechanics:
Path of Exile has six different character classes: Witch (INT), Shadow (INT/DEX), Ranger (DEX), Dualist (DEX/STR), Marauder(STR) and Templar(STR/INT) encompassing three main attributes; Intelligence, Dexterity and Strength.
Skills: Path of Exile has been designed so that any class can use any skill if that's what the player decides. As long as the skill requirements are met, it can be used. Skills are are actually items which will also level up and get stronger as long as they are equipped. To use a skill, you must first socket it into an item. All items (excluding belts, quivers, rings, amulets and flasks) have sockets in which skills can be equipped. If the item has two or more sockets that are linked together, any skill within those sockets can be augmented to add additional effects to make it stronger. Example, a Fireball skill could be linked to Lesser Multiple Projectiles meaning that 3 fireballs could be launched instead of one when the skill is used. More info on skills can be found here.
Passive skill tree: Every time your character levels up, you will gain a passive skill point. This allows you to pick a passive benefit from one of 1400 nodes essentially allowing any kind of build! Path of Exiles passive skill tree.
Currency: This ARPG doesn't have a normal gold based currency system because there is no gold. Instead players find Orbs which have an actual in game use. Because of this each orb is more or less assigned a value by the players themselves based on rarity and actual usefulness. This also creates a barter economy. All currency items can be found here.
Items: As per all other ARPG's, Path of Exile follows a similar item system with Normal (white) items with only implicit magical mods, followed by magical items with up to two magical effects, followed by rare items with up to seven magical effects. Finally there are unique items which have a low drop rate and some magical effects/large magical effect values unique to the item (like always hit, blood magic, +40-60% increased item rarity or just +100 to all attributes). This also means that high level rare items can always be found to be better than most uniques. Path of Exile does not have set items.
Flasks: Instead of requiring many health/mana pots to keep your character functioning, Path of Exile incorporates flasks which refill automatically as you kill monsters. Flasks can also have magical effects that either make the flask more effective or apply extra effects while the flask is in use.
Leagues: Path of Exile has a Default league, in which characters respawn when they die and a Hardcore league, in which when characters die they are sent to the Default league with their currently equipped items. Cutthroat league events are PvP where other players can 'invade' another players instance and kill them for all of their items.
So, how do I play?
Path of Exile is currently in closed beta and the only way(s) to get beta access is to sign up and hope you get picked (an account is given beta access every 5 minutes) or to support Grinding Gear Games and purchase a supporter pack which will also give you access.
Path of Exile is expected to enter open beta later this year.

