As the name implies, multithreading is the act of using multiple threads in a program.

When writing multithreaded programs, you need to be careful with concurrent access. This means two threads accessing the same variable at the same time.

You can avoid concurrency issues by using the lock keyword. It locks a resource (variable) so other threads that try to access it will have to wait until the first thread is done with it's work.

Example:

int x = 0;

void Add1()
{
	//any object can be locked, really
	//usually a dedicated "lock" object is created
	lock(x) 
	{
		x = x + 1;
	}
}

void Main()
{
	Thread myThread1 = new Thread(new ThreadStart(Add1));
	Thread myThread2 = new Thread(new ThreadStart(Add1));
	myThread1.Start();
	myThread2.Start();
	//x will be 2 here.
}

There is one big problem with locking, and that is when Thread1 is waiting for Thread2 and Thread2 is waiting for Thread1. This is known as deadlock.