Assign is for primitive values like BOOL, NSInteger or double. For objects use retain or copy, depending on if you want to keep a reference to the original object or make a copy of it. 

assign: In your setter method for the property, there is a simple assignment of your instance variable to the new value, Ex:

(void)setString:(NSString*)newString {
 string = newString;
}

This can cause problems since Objective-C objects use reference counting, and therefore by not retaining the object, there is a chance that the string could be deallocated whilst you are still using it.

retain: this retains the new value in your setter method. For example:

This is safer, since you explicitly state that you want to maintain a reference of the object, and you must release it before it will be deallocated.

(void)setString:(NSString*)newString {
 [newString retain];
 [string release];
 string = newString;
}

copy: this makes a copy of the string in your setter method:

This is often used with strings, since making a copy of the original object ensures that it is not changed whilst you are using it.

(void)setString:(NSString*)newString {
 if(string!=newString){
 [string release];
 string = [newString copy];
 }
}