To most people, there's people in the know, like yourself, and people who haven't the faintest idea how you'd even begin to go about doing something like breaking into NASA computers. Well, generally speaking, you would have to have a high level understanding of how networks operate and then you would have a high level understanding of programming and vulnerability of computer systems. There are all kinds of back doors you can use to get in, depending on what the administrators of computer networks leave open. A lot of hacking, or network intrusion, is just bad management of the network. It's like having a house and forgetting to go around and lock the back door and close the windows and make sure that the windows going into the basement are shut.
Back door is a common term.
It depends where you get in. Alternatively you could work your way up the security chain. Or you could take over a system. Usually what a lot of people want to do is just get in there and leave by a back door so that they can get in and out whenever they want. What others want is a stepping stone to another computer, so that they can hack into one system without revealing their tracks.