Question

Are selectors in Objective-C just another way to send a message to an object? I really don’t understand why or how to use them.

Answer 1

They aren’t another way to send a message to an object, they’re the only way. For example, in [myView setValue:@"foo"], setValue: is a selector. (Another, less convenient way of writing the same thing is objc_msgSend(myView, @selector(setValue:), @"foo").)

As Ian Henry says, you can use SEL values to choose a selector at runtime instead of compile time. This is a fundamental technique in Cocoa; user interfaces are generally connected to controllers using target/action bindings, where the target is an object and the action is a selector. Normally you set this up in a nib, but you can also do it in code:

[myButton setTarget:myController];
[myButton setAction:@selector(buttonClicked:)]; // Clicking the button will now call [myController buttonClick:myButton].