CPUs, parallelism, and threads

I am using SQL Server 2008 hosted on Windows Server 2008R2 – 64bit. Our system has four CPUs. I have the following questions about CPUS in relation to parallelism and threads.

1. In parallel execution does it follow that a thread handles one part of the execution, what I mean is – If there are 10 threads (for example) displayed on Windows Task Manger while a query is running, does it mean that the execution of that query will have been split into 10 parallel parts?
2. OR is the degree of parallelism determined by the number of the available CPUs? Like in my case with four CPUs – does it mean that query parallel execution cannot be split into more than four parts?
3. What’s the purpose of a thread anyway when a query runs in relation to parallelism? How does that tie in with CPUs in use?

Hope my question is clear.

Many thanks,

Mpumelelo