Objective-C's "obj performSelector" vs objc_msgSend( )?

Going through Apache Cordova's source code, I ran into two lines of code that I'm puzzled about:

    //[obj performSelector:normalSelector withObject:command];
    objc_msgSend(obj,normalSelector,command);

From Apple's documentation, there doesn't seem to be a lot of difference between these two methods.

id objc_msgSend(id theReceiver, SEL theSelector, ...) Sends a message with a simple return value to an instance of a class.

- (id)performSelector:(SEL)aSelectorwithObject:(id)anObject Sends a message to the receiver with an object as the argument. (required)

What exactly is the difference between these two methods? In the case above, both are sending messages with an object as an argument to a receiving object.