Both threads and processes are methods of parallelizing an application. However, processes are independent execution units that contain their own state information, use their own address spaces, and only interact with each other via interprocess communication mechanisms (generally managed by the operating system). Applications are typically divided into processes during the design phase, and a master process explicitly spawns sub-processes when it makes sense to logically separate significant application functionality. Processes, in other words, are an architectural construct.
By contrast, a thread is a coding construct that doesn't affect the architecture of an application. A single process might contains multiple threads; all threads within a process share the same state and same memory space, and can communicate with each other directly, because they share the same variables.
Threads typically are spawned for a short-term benefit that is usually visualized as a serial task, but which doesn't have to be performed in a linear manner (such as performing a complex mathematical computation using parallelism, or initializing a large matrix), and then are absorbed when no longer required. The scope of a thread is within a specific code module—which is why we can bolt-on threading without affecting the broader application.
--Alan Zeichick
Read the rest in Ready for Threading: Five Easy Steps for Making Your Code Run Faster