Apps are the lifeblood of Snarl: they provide the source oxygen that keeps it alive. From fun stuff like People in Space (which, surprisingly, notifies you when someone has jetted off into – or returned from – Space) and Mars Weather (which, amazingly, notifies when the weather conditions on Mars change) to more mainstream applications like Handbrake and Cyberduck.
Ultimately: apps generate notifications. These can be local applications running on your PC, an application running on a server on your network, or SaaS applications running in the Cloud. You then define how Snarl presents these notifications to you.
Services display notifications. This might be on-screen in a balloon or toast way; or it may be via more subtle routes – for example, changing the colour of an LED, or by notifying you through a Cloud-based web service.