Purpose of @ Symbol Before Strings?

I've been using Objective-C for a while now, but have never really understood what the purpose of the @ symbol before all strings is. For instance, why do you have to declare a string like this:

NSString *string = @"This is a string";

and not like this:

NSString *anotherString = "This is another string";

as you do in Java or so many other programming languages. Is there a good reason?