In many Books and on many Sites I see -drain. Well, for an Autorelease Pool that sounds cool. But does it do anything other than an release? I would guess -drain just makes the Pool to -release all it's objects, without releasing the Pool itself. Just a guess.
Note that the comments on oxigen's answer saying that -drain does not release the NSAutoreleasePool are not correct. The documentation for NSAutoreleasePool clearly says that -drain releases (and thus destroys) the NSAutoreleasePool.
-drain is a replacement for using -release for NSAutoreleasePool objects, the only difference being that provides a hint to the garbage collection system.
If your system has a garbage Collection, then -drain send message (objc_collect_if_needed) for GC
If you haven't GC, then drain = release
Oxigen is right, see the documentation for method drain of NSAutoreleasePool:
In a reference-counted environment, releases and pops the receiver; in a garbage-collected environment, triggers garbage collection if the memory allocated since the last collection is greater than the current threshold.
来源：https://stackoverflow.com/questions/797419/whats-the-difference-between-sending-release-or-drain-to-an-autorelease-pool