The memory space, where a given application is executed is called - process. A Process is the memory set aside for an application to be executed in.
Within this process the thing, which is really executed is the thread.
The key difference is that processes are fully isolated from each other; threads share (heap) memory with other threads running in the same application.
Threads share the address space of the process that created it; processes have their own address.
Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.
Threads can directly communicate with other threads of its process; processes must use inter-process communication to communicate with sibling processes.
Threads have almost no overhead; processes have considerable overhead.
New threads are easily created; new processes require duplication of the parent process.
Threads can exercise considerable control over threads of the same process; processes can only exercise control over child processes.
