On an operating system running program is known as process,process are generally called heavy weight process.

A single process can have separate runnabletask called threads.a thread is a single sequential flow of control within a process  a thread is also referred to as a lightweight process 

A process has a self-contained execution environment. A process generally has a complete, private set of basic run-time resources; in particular, each process has its own memory space. 

Threads are sometimes called lightweight processes. Both processes and threads provide an execution environment, but creating a new thread requires fewer resources than creating a new process