My new phone came with super user but I didn't root it I've just got a new Samsung Galaxy Y from Unique Mobiles 
(http://www.uniquemobiles.com.au). The package was still intact when I picked it up and the "don't accept this if this sticker is broken" sticker was still good as new. Now I'm new to the Android operating system (coming from an iPhone) and when I got my phone it had superuser already on it. I had no idea what it was so I just left it alone. Until a friend from school asked if I had rooted it, I didn't have a clue to what he was saying. He said the only way I can get it is by rooting it. I haven't given my phone to anyone to do anything with it, I've been the only one to use it and I'm 100% sure I haven't rooted it.

Now my big question is 
How did it get there?
What does it do? 
Has anyone else had the same thing happen to them?

I was also just wondering about some more information on superuser, rooting and ways that it could have gotten onto my phone.
I'd check with the operator, and see if they root their phones by default (it's a perfectly valid thing to do, it's just that the big operators tend to be control freaks). If they don't know what you are talking about, worry and ask them for help figuring out what's going on.

I'd also go into SuperUser and check it's preferences. Since you don't know how it got there, you don't know what apps might be doing. Turn on it's logging (if not already enabled) and set 'Automatic response' to 'prompt'. That way you'll have a log of what's using it, and you'll be prompted for anything that wants to do things with root access. The idea is that if something asks for root access and you don't know why, just deny it and see what breaks - very few apps should need root access, so it shouldn't come up very often.

If logging is already enabled, then you can review the log to see if any apps have been using root access.