iOS had restricted multitasking exposed to apps for awhile (definitely by iOS 5) but it was restricted to certain kinds of apps: VOIP, background audio, newsstand, location awareness, or to a restricted period of time after the app is backgrounded.
They expanded it quite a bit in iOS 7? to all apps with background fetch, background url upload and download tasks, silent push notifications, and background tasks.
All of these approaches do not work if a user forcibly shuts down the app (I am not quite sure of every case). All these mechanisms are controlled by the OS: we'll call you, don't call us sort of thing and if you don't return in a certain period of time or if we need to, we will shut you down.
In iOS 8 you have extension support which launches mini apps. Extensions are somewhat equivalent to services but they are about integration with multitasking an implementation detail for security purposes.
They expanded it quite a bit in iOS 7? to all apps with background fetch, background url upload and download tasks, silent push notifications, and background tasks.
All of these approaches do not work if a user forcibly shuts down the app (I am not quite sure of every case). All these mechanisms are controlled by the OS: we'll call you, don't call us sort of thing and if you don't return in a certain period of time or if we need to, we will shut you down.
In iOS 8 you have extension support which launches mini apps. Extensions are somewhat equivalent to services but they are about integration with multitasking an implementation detail for security purposes.