I just copied and pasted a couple of section from their faqs:
“Joost™ is a new way of watching TV on the internet, which uses new and established technologies to provide the best of both the internet and TV worlds. We’re in the process of making it as TV-like as we can, with programmes, channels and adverts. You can also see some things that we think will enhance the TV experience: searching for programmes and channels, for example, as well as social features like chat. There are many more new features to come!
Currently, channels are like playlists of videos. You can flip between channels, or use the programme guide to choose a channel or a video to watch. The channels start when you play them and not at any particular day or time. If you let them continue they will loop round to the beginning again. If you change channel and then switch back again, you’ll start at the beginning of the programme again. In later versions we will have different kinds of channels, including ones that behave more like ordinary TV channels. Occasionally, you may see an advert.“