Here's the official gameplay video for Grand Theft Auto Online.

Official overview from Rockstar:

Grand Theft Auto Online is a dynamic and persistent online world for 16 players that begins by sharing gameplay features, geography and mechanics with Grand Theft Auto V, but will continue to expand and evolve after its launch with new content created by Rockstar Games and the Grand Theft Auto community. Grand Theft Auto Online takes the fundamental Grand Theft Auto concepts of freedom, ambient activity and mission-based gameplay and makes them available to multiple players in an incredibly detailed and responsive online world.

In Grand Theft Auto Online, players have the freedom to explore alone or with friends, work cooperatively to complete missions, band together to participate in activities and ambient events, or compete in traditional game modes with the entire community, all with the personality and refined mechanics of Grand Theft Auto V.

Players can invest in their character through customizing their appearance, improving their stats, owning customized vehicles, purchasing personal property, and taking part in missions, jobs and activities to earn reputation and cash to open up new opportunities to rise through the criminal ranks. The world of Grand Theft Auto Online will constantly grow and change as new content is added, creating the first ever persistent and dynamic Grand Theft Auto game world.

Access to Grand Theft Auto Online is free with every retail copy of Grand Theft Auto V and launches on October 1st.