http://www.giantbomb.com/grand-theft-auto-v/3030-36765/forums/lots-of-new-gta-online-info-released-from-cvg-1449039/

CVG got 50 minutes hands-on with GTA Online and recounted their experience here. I made a list of the info:

Characters from GTA V will appear in GTA Online
GTA Online will not always be limited to Los Santos; additional locations are in the works

CVG says you can do anything in GTA Online that you can do in GTA V. They list the following: heists, co-op missions, gang attacks, death matches, impromptu deathmatch, races, tennis, golf, base jumping, shooting range, ambient events (hold ups of stores, armored van robberies, crate drops, import/export missions involving car theft, bounties that you can set on other players and people can claim), betting
If you are killed while holding money (CVG says 1million as an example) you will lose a large portion of it, and others can steal it. Use banks to keep money safe

An in-game smartphone can call your friends whether they are in GTA Online or GTA V. You can also use this to set waypoints for your friends. While using the phone, your avatar moves their lips with yours.

While you are driving, you can choose to grant someone else permission over the radio
In-game AI will respond to how aggressive you are on your mic (as in, if you're shouting during a robbery, the AI will react and speed up/become more frightened)

Whoever sets up a heist decides who gets what cut of the final money when that money is obtained

Garages range from two car ports to 10 car ports

Cars can have retractable roofs controlled by the player

Missions can be accessed by physical markers or through a quickmatch sort of menu

User-made missions can be favorited and rated

You can assign other players certain roles in missions based on the particular abilities they have leveled up

You can share cash and ammunition with other players

There are stealth kill animations, possibly with a knife. The game tracks the amount of noise your character is making, and the AI will respond accordingly
You can steal physical items. During the demo, a mission was done in which a bike warehouse was robbed. The bikes had to be transported out in a large truck
"RP" are the points you get to level up your character

You can use physical gestures, such as flipping the bird

All of your info/communications in the game appear in the "Feed." Rockstar mentioned this as a sort of Twitter for GTA Online

You can purchase insurance for your car. By insuring your vehicle, if it gets destroyed or stolen, you can have it replaced by paying a small premium

Police helicopters have cameras mounted to the front of them. In real time, you can watch on live TV (on the news channel in your apartment) players with a high wanted level being chased by the police. There are additional channels with programs such as a crime noir show

Heists can have up to 20 different components. A heist is mentioned where a Titan (largest aircraft) is guarded by "Maryweather" (sp?). The Titan is indeed a flyable aircraft

There is a heli-pad on the roof of (at least) the largest AmmuNation and the store can also be accessed from there

Off the west coast of Los Santos, there is an another island with lots of buildings. This has not been seen before, but the folks at CVG saw it from afar

There is a "passive" mode where you cannot shoot or be shot