Here's some info I put together both from the Official Website and myself on what we know about Starcraft II so far.

PROTOSS

Returning units/buildings:

Zealot, Dark Templar, Gateway/Warp gate, and the Stargate.

New Units:

Immortal - Basically, the new dragoon. It is refitted with twin phase disruptors and hardened energy shields that can shrug off the most powerful weapon strikes - though at the cost of leaving them more vulnerable to the pinprick attacks of lesser foes.

Stalker - Basically a mix of Dark Templar and dragoons. The stalker is a machine controlled by the shadow-essence of a dark templar warrior fused into a metal body to protect his people. One of the greatest powers it is credited with is the ability to instantly teleport or 'blink' from one spot to another.

Phase Prism - This is basically a mix between an Arbiter and a moving Pylon. The primary function of a phase prism is to transport protoss forces on the battlefield. It can also power protoss structures around it, temporarily replacing destroyed pylons.

Phoenix - Mix between scouts and corsairs. They are generally weak, but have the ability to attack with an "overload ability". The Power Overload is devastating and can destroy a large number of enemies at once, but it comes at a price: in the aftermath the phoenix is left temporarily powerless and crippled.

Colossus - A towering spider-like machine which strides across the battlefield on four tall, slender legs which give it an extremely high level of mobility. The great height of the machine allows it to step up and down cliffs and other steep elevations with little difficulty, and gives it a dominating field of fire. It shoots 2 powerful thermal beams or lances. As a target is destroyed, the lances are switched to a new target instantly so that a colossus' beam cuts a fiery swathe of destruction through multiple enemies with each firing cycle.

Warp Ray - Don't get this confused with the Phase Prism. This doesn't actually warp units, but has a long ray beam that the longer it's locked on the target, more of the focusing array becomes aligned, and up to three individual power streams combine together. Over a few seconds the beam's intensity and power grow exponentially.

Twilight Archon - Created from merging any 2 templar (a high and a dark templar, or 2 dark, or 2 high), this basically takes place of the old Archons. Their devastating psionic storms can be unleashed against cowering enemy forces both in the air and on the ground.

TERRAN

Returning units/buildings:

Marines, Ghosts (now with sniping ability), Command Center. (More to come such as Battlecruisers, as shown in videos)

New Units:

Reapers - These are like Marines on steroids, or long term stim packs. Except these units also include a jet pack, giving them a high degree of mobility, including the ability to scramble up or down cliffs and jump across rivers, crevasses, and other obstacles. Reaper combat techniques emphasize hit-and-run raiding, preferably against enemy structures and workers, so that reapers can inflict maximum damage and then retreat before the enemy can muster a response.

Viking - A mix of Wraiths, Valkyries, and Goliaths. The viking was designed to be the ultimate anti-air and ground-support weapon system. With the ability to change its combat role from an assault walker to an air-superiority fighter, the viking can switch smoothly to fulfill tactical needs in a developing battle.

Info from BlizzCon 2007: Hands on with SC:2

http://tinyurl.com/2f83oz

First look at Single Player Campaign:

http://tinyurl.com/2ohof4

Search YouTube for great GamePlay Videos as well.

COMING SOON: More units and the Zerg race details.