Custom Notification System using Collections

How would I go about creating a custom notification system for my site members? I can picture the notification system being connected to a database. Certain actions on the site like forms being completed, announcements/updates, etc. would be in the notification center. How would I do this so that users see their own notifications?
Right now I have a database that have very general notifications as all the site members can see them. Would I filter the dataset by Owner? If yes, then how would could I include notifications that go out to everyone? Open to all ideas.