What does it mean when legal records are "sealed"?

I got to thinking about this because a friend of mine got a DUI a few months ago, and is going through all the hoops of the punishment. After everything is complete, after a year, since it’s the first time, by Kansas law the record will be expunged or deferred or whatever. Her attorney said it will be ‘like it never happened, unless it happens again.’ Of course, the record will still be out there, or they wouldn’t know if it happened “again.” So what does that mean, and how accessible are these records?

Then I also remembered that a couple of years ago I was at the court house looking up some records on an individual, and found an old charge of evading the police…but it happened when he was a minor. I thought charges against minors were sealed, unless something major happened to unseal them.

So are those things available to the general public or not? How hard is it to get information that, by law, you don’t have to reveal?