I've searched a bit on Google without luck, i don't know if this kind of extensions has a specific naming in Umbraco-land? I want to restrict a specific user-group to only be able to read and edit content that they themself have created. I guess the necessary info already lives on a content node, but how would one go about extending Umbraco to allow for this kind of policy?

It's not a feature within Umbraco, but you could probably do something in an EditorModelNotification