Each process is started with a primary thread, but can create additional threads when required
Process run in separate address space, but threads of a process share address space4
Threads have direct access to data segment of its process; process have their own copy of data segment of parent process. refer to point #2
Threads can directly communicate with other threads of that process; processes must use inter-process communication to communicate with sibling processes, directly follows from point #2.
Threads carry less state information than process; thus they incur lower overhead compared to processes.
New threads are easily created; new process require duplication of parent process, and allocation of memory and resources for it are costly.
Changes to main thread [cancellation, priority change] may affect behavior of other threads of that process; changes to parent process does not affect child or peer processes
Context switching between threads in the same process is typically faster than context switching between processes.
In certain OSes, a thread is also known as a lightweight process(lwp), Solaris is one such OS.