Like @Cyprian said a bundle is a set of functionality that can work alone. As it happens while developing we do not always know when things are separate. It comes with time.
Personally, I am working with Symfony2 since Febuary and I never stopped reading the manual and related books to understand more in depth. That helped a lot and is very interesting read, I assure you :)
Here is my top favourites documentation pages, en enlightening blog posts on delicious.
For your immediate question, forget about "frontend" and "backend" as we were doing in symfony 1.x. Just think of Model entities (as in A SINGLE row) and build in one bundle. As your code will grow, you will see how to disassemble and separate in bundles. You just have to keep in mind to separate your functionnalities in small methods and refactor around.