It's been announced that the replay feature that was introduced to TF2 not so long ago will be added to the CS:S beta build on August 30th.

The feature allows players to record their actions including all player perspectives rather than just their point of view, which will make reviewing in game decisions easier and will be a boon for movie makers.

It was explained on the Steam forums thusly:

At its core, Replay is an extension of SourceTV. The feature must be enabled on the server side, and the "demo" files that are ultimately generated include the full PVS, meaning everyone's perspective is recorded. Regular demos, on the other hand, only record the local player's perspective.

At the beginning of the "setup" phase for a round of gameplay, the Replay system will begin recording if enabled (via the replay_enable ConVar). The round, also known as a recording session, is given a unique name based on the date and time of the recording. As the round/recording session progresses, small blocks of data are periodically published to a fileserver where they can be downloaded by clients.

If you want to know more read the whole FAQ, which can be found here.