In the analog world, if a burglar is going to rob your house, he follows several predictable steps:
He targets your house in particular.
He checks the locks on your doors and windows to find an open one or uses a special tool to get in.
He breaks in.
He moves quietly from room to room until he finds your valuables.
He leaves, just as quietly.
As we'll see, hackers have the same modus operandi.