Not including games are there really only 4 classes of apps?
1. Fixed content apps
A fixed body of content is displayed in the app. The style of presentation is often very compelling. For example:
The knot app
2. Task apps
The app connects to a service either on the phone or in the network that enables it to do something. For example:
A spirit level app
A webcam viewer app eg: my webcam
3. Social apps
The users supply the content. For example:
A twitter client
A photo sharing app
4. Combinations of 1,2,3
A combination of classes 1,2 and 3 above. For example; a GPS app that enables you and friends to meet at a McDonalds nearby.