A thread is the execution of a piece of code inside a process. Multiple threads can execute simultaneously inside a process. They are put, altogether, inside an execution priority queue by the operating system, then are run by small groups on the cores of the processor. See Figure 1.
Figure 1:Thread execution (from Wikipedia)
Threads:
Are part of the same process.
Share the same memory inside their process.
With threading we can design applications that use:
Concurrent programming.
parallel programming.
both.
In concurrent programming:
Threads share the same resources (memory data, files, ...). They are in a race condition: they need to access resources in order to execute.
We need to coordinate the threads in order to avoid deadlock (two threads that wait for each other indefinitely).
In parallel programming:
Threads access different resources (memory data, files, ...).
The resources must be shared equally in order to achieve the best speed gain.