What is use of performSelector in iOS

What is the role of performSelector?

Comparing:

[self btnClicked];

and

[self performSelector:@selector(btnClicked)];

-(void)btnClicked
{
    NSLog(@"Method Called");
}

both are woking fine for me. What is difference between these two. [self btnClicked] and [self performSelector:@selector(btnClicked)];