Understanding Java annotations

I've looked around a bit, and I'm failing to understand parts of how Java annotation work.

All the examples I've seen so far create the annotation, then have a main method that runs through the classes in the project using reflection. Then they will do stuff with that and basically make the annotation work.

However I'm failing to understand how this works if I want to make my annotation project into a jar that I can include in another project, like jackson, guice, hibernate, etc. A main method wouldn't work in this case, right?

I've looked for some tutorials on how to make annotation that is done as an jar that my project can include, but I haven't found anything yet. Ideally I'd like to be able to use it inside a web framework such as spring or play.