Some assignments are take-home tests or other projects that you are obviously not supposed to use the Internet for. I think if you know you're not supposed to receive outside help and you choose to, that's cheating. However, if you didn't know then I'm not sure. Always clarify with the teacher ^.^

If you need to just give facts in your assignment, and you get them from the internet - then obviously yes, it is cheating. But if you need to find something, and then analyze or interpret it - then it's called research, and is actually encouraged by most teachers.

If you are to do research - then the internet use is not cheating. If you are doing a test - that is not "open book", then using the internet is definitely cheating.