Joost is a free, full-screen television application that you download to your PC. The programming guide and on-demand controls behave much the same as does a cable set-top box. An embedded web browser can be brought up for more detail on a particular program.
Interestingly, every piece of content in the system has its own unique, referencable URL, so every pico can be tagged, annotated, or commented on. Users can also create and share their own overlays. This implies that Joost will be able to get around copyright act issues by not editing content, but by allowing users to do what they wish around the content. (Cuts takes a similar approach to sharing video edits - they affect playback rather than the content itself.)
The company is currently struggling with how to help users discover content. The list of channels is just that, one really long list that pops up on the left side of the screen. There appears to be plenty of metadata available, but an easily browsable interface is still a work in progress. Presuming that Joost is successful enough to get full mainstream content - e.g., it becomes the next DirecTV - this will be critical.
The product is in a private beta currently, so cozy up to a friend who attended Web 2.0 Expo -- he or she may have an extra invite if you'd like to take it for a spin.