Multithreading allows two parts of the same program to run 
concurrently

A thread executes a series of instructions. Every line of 
code that is executed is done so by a thread. Some threads 
can run for the entire life of the applet, while others are 
alive for only a few milliseconds. 

Multithreading is the ability to have various parts of 
program perform program steps seemingly at the same time. 
Java let programs interleave multiple program steps through 
the use of threads. For example,one thread controls an 
animation, while another does a computation. In Java, 
multithreading is not only powerful, it is also easy to 
implement. 

You can implement threads within a java program in two 
ways – Creating an object that extends Class Thread or 
implementing the interface Runnable.

The key difference between the two is that Thread class has 
a strart() method which your program simple calls whereas 
the Runnable class does not have a start method and you 
must create a Thread object and pass your thread to its 
constructor method. You never call run() method directly; 
the start method calls it for you.

Example:

Class MyProcess extends Thread{
	Public void run(){
}
	public static void main(String args[]){
		MyProcess mp = new MyProcess();
		mp.start();
}
}
Class MyProcess implements Runnable {
Public void run(){
}
public static void main(String args[]){
		Thread mp = new Thread(new MyProcess());
		mp.start();
}
}

at first ,let us know what is thread . Thread is also a functionality which executes concurrently with other of the program ,unlike general function.
Now let us see what is multithreading , executing mutiple thread simultaneously is called as multithreading.

thread is the process of execution of program is called 
thread 



multithread means when sub programs is execute on parallel 
at same time