The render farm relies on you to live; the more computers are connected to the system, the better the service will be.
You don't need to have Blender installed, it will be downloaded for you.
Your own project will be rendered first, then your computer will help other users.
If you don't want to share your computer, but still help SheepIt, you should consider donating.
This is a small FAQ about launching the client, the FAQ page contains more general information about the service.