The standard OpenStep system of memory management employs retain counts. When an object is created, it has a retain count of 1. When an object is retained, the retain count is incremented. When it is released the retain count is decremented, and when the retain count goes to zero the object gets deallocated.

A simple retain/release mechanism has problems with passing objects from one scope to another, so it's augmented with autorelease pools. You can use the AUTORELEASE() macro to call the [NSObject -autorelease] method, which adds an object to the current autorelease pool by calling [NSAutoreleasePool +addObject:] .
An autorelease pool simply maintains a reference to each object added to it, and for each addition, the autorelease pool will call the [NSObject -release] method of the object when the pool is released. So doing an AUTORELEASE() is just the same as doing a RELEASE() , but deferred until the current autorelease pool is deallocated.

The NSAutoreleasePool class maintains a separate stack of autorelease pools objects in each thread.

When an autorelease pool is created, it is automatically added to the stack of pools in the thread.

When a pool is destroyed, it (and any pool later in the stack) is removed from the stack.

This mechanism provides a simple but controllable and reasonably efficient way of managing temporary objects. An object can be autoreleased and then passed around and used until the topmost pool in the stack is destroyed.

Most methods return objects which are either owned by autorelease pools or by the receiver of the method, so the lifetime of the returned object can be assumed to be the shorter of the lifetime of the current autorelease pool, or that of the receiver on which the method was called.
The exceptions to this are those object returned by -

[NSObject +alloc] , [NSObject +allocWithZone:]

Methods whose names begin with alloc return an uninitialised object, owned by the caller.

[NSObject -init]

Methods whose names begin with init return an initialised version of the receiving object, owned by the caller.
NB. The returned object may not actually be the same as the receiver... sometimes an init method releases the original receiver and returns an alternative.

[NSObject +new]

Methods whose names begin with new combine the effects of allocation and initialisation.

[NSObject -copy] , [<NSCopying>-copyWithZone:]

Methods whose names begin with copy create a copy of the receiver which is owned by the caller.

[NSObject -mutableCopy] , [<NSMutableCopying>-mutableCopyWithZone:]

Methods whose names begin with mutableCopy create a copy of the receiver which is owned by the caller.