Naming conventions: Guidelines for verbs/nouns and english grammar usage

Interfaces are things a class is capable of doing. Not what it is, but what it can do.

IGroupableItem

Other names describe what things are or are too vague to be useful.

Specifically, "IDataEntity" is largely meaningless. After all, everything's a data entity.

Look at the MSDN articles for naming guidelines. In short:

• Use nouns for class names and property names (it's obvious)
• For interface names, start with I and use nouns and/or adjectives to describe behavior
• Use verbs for method names to describe action

For your example - IGroupableItem.

MSDN has an article just on Interface Naming Guidelines that may help you out. If you want the naming conventions of stuff other than interfaces, along with many other naming and design guidelines, you can find that all on MSDN, too.