Short answer: if you’re using an NSString you should be using copy.

http://stackoverflow.com/a/387984/343204

Long answer

Copy makes a copy of a variable.

Retain shifts the pointer when set to a new variable. i.e. it becomes the new variable.

Here’s an example that clarifies whether to use retain or copy:

NSMutableString *someName = [NSMutableString stringWithString:@”Dougal”];

Person *p = [[[Person alloc] init] autorelease];
p.name = someName;

[someName setString:@”Zebedee”];

If p is declared using copy then it’s set to Dougal on line 3 and, on completion, still has that value.

However, if p is declared using retain then it’s set to someName. So, when someName is set to “Zebedee”, p is also set to “Zebedee”.

More on this here: http://stackoverflow.com/questions/387959/nsstring-property-copy-or-retain

And if you want to get into the details of implementing NSCopying or NSCopyObject then you should look at Rob Napier’s post: http://robnapier.net/blog/implementing-nscopying-439