A process is a program in execution, and a thread is just a CPU state stored within a process. A CPU context is either a process or a thread.
Most processes have only one thread, but some processes, particularly servers, have more.
These processes and threads represent the programs the user has selected to run. If managing processes and threads is slow, overall computer performance will also be slow. Linux advocates naturally assume that Linux offers better performance than MS Windows. His results generated quite a controversy in the Linux newsgroups and the kernel e-mail list. All tests were done on a 200 MHz Pentium.