The default/implicit values are atomic, readwrite, and assign.
atomic
This means that the value is read/written atomically. Contrary to the somewhat popular misconception, atomicity does not equate to thread safety. In simple terms, it guarantees that the value you read or write will be read or written in whole (when the accessors are used). Even when you use accessors all the time, it’s not strictly thread safe.
readwrite
The property is given a setter and a getter.
assign
This default is usually seen used for POD (Plain-Old-Data) and builtin types (e.g. int).
For NSObject types, you will favor holding a strong reference. In the majority of cases, you will declare the property copy, strong, or retain. assign performs no reference count operations. See also: http://clang.llvm.org/docs/AutomaticReferenceCounting.html#property-declarations
strong
The property may be implicitly strong under ARC in some cases:
A property of retainable object pointer type which is synthesized without a source of ownership has the ownership of its associated instance variable, if it already exists; otherwise, [beginning Apple 3.1, LLVM 3.1] its ownership is implicitly strong. Prior to this revision, it was ill-formed to synthesize such a property.