how to instantiate an object of class from string in Objective-C?

1) what does NSClassFromString return??

It returns a Class, which means you can do [[NSClassFromString(@"MyClass") alloc] init]; see Mac Dev Center Docs for more info.

2) what is id?

See http://www.otierney.net/objective-c.html#idtype

3) How to call method -(void)FunctioninClass which is in MyClass using the instance.

id myclass = [[NSClassFromString(@"MyClass") alloc] init];
[myclass FunctioninClass];