what is Selector in iOS / what is Selector in iPhone

 A selector is the name used to select a method to execute for an object, or the unique identifier that replaces the name when the source code is compiled.  A selector by itself doesn’t do anything. It simply identifies a method. compiler makes sure that selectors are unique.

What makes a selector useful is that (in conjunction with the runtime) it acts like a dynamic function pointer that, for a given name, automatically points to the implementation of a method appropriate for whichever class it’s used with.

Getting a Selector

At compile time, you use the compiler directive @selector.

SEL aSelector = @selector(methodName);

SEL aSelector = NSSelectorFromString(@"methodName");

Using a Selector

SEL aSelector = @selector(run);

[aDog performSelector:aSelector];

[anAthlete performSelector:aSelector];

[aComputerSimulation performSelector:aSelector];

At runtime, you use the NSSelectorFromString function, where the string is the name of the method:

You use a selector created from a string when you want your code to send a message whose name you may not know until runtime.

Compiled selectors are of type SEL. There are two common ways to get a selector:

You can invoke a method using a selector with performSelector: and other similar methods.

(You use this technique in special situations, such as when you implement an object that uses the target-action design pattern. Normally, you simply invoke the method directly.)