Property

Here's the short answer:

atomic vs nonatomic primarily ensures that complete values are returned from synthesized getters and that complete values are written by synthesized setters.

readwrite vs readonly determines whether a synthesized property has a synthesized accessor or not (readwrite has a setter and is the default, readonly does not).

assign vs retain vs copy determines how the synthesized accessors interact with the Objective-C memory management scheme:

• assign is the default and simply performs a variable assignment
• retain specifies the new value should be sent -retain on assignment and the old value sent -release
• copy specifies the new value should be sent -copy on assignment and the old value sent -release.

http://stackoverflow.com/questions/7085160/what-is-the-difference-between-a-readwrite-property-and-a-nonatomic-assign-prope

Nonatomic

nonatomic is used for multi threading purposes. If we have set the nonatomic attribute at the time of declaration, then any other thread wanting access to that object can access it and give results in respect to multi-threading.

Copy

copy is required when the object is mutable. Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.

Retain

retain is required when the attribute is a pointer to an object. The setter generated by @synthesize will retain (aka add a retain count to) the object. You will need to release the object when you are finished with it. By using retain it will increase the retain count and occupy memory in autorelease pool.

Strong

strong is a replacement for the retain attribute, as part of Objective-C Automated Reference Counting (ARC). In non-ARC code it's just a synonym for retain.

This is a good website to learn about strong and weak for iOS 5.http://www.raywenderlich.com/5677/beginning-arc-in-ios-5-part-1

Weak

weak is similar to strong except that it won't increase the reference count by 1. It does not become an owner of that object but just holds a reference to it. If the object's reference count drops to 0, even though you may still be pointing to it here, it will be deallocated from memory.

The above link contain both Good information regarding Weak and Strong.

http://stackoverflow.com/questions/9859719/xcode-property-attributes-nonatomic-copy-strong-weak