Multithreading concept in operating systems

To understand the concept of multithreading first understand the concept of procedural language....for example in C language you write a program...and you write functions or procedures for every individual task...the same concept is with the multithreading in operating systems...In operating systems we have processes...and each process can be divided into threads...it is a way in which a process can break itself into one or more simultanoeusly running tasks,every process have some resources now these threads of that process share the same resources...
In single core processors the multithreading occurs in time division multiplexing..means each thread get some time slice to execute.this execution time is very small such that it seems to be executing simultaneously.in multiple core processors..the threads work on different processors here they are actually executed simultaneously.The operating systems like Windows NT and OS/2 are said to have "cheap threads" and "expensive processes".