It's not extended per se. It's the US theatrical cut. It wasn't well received there, and Kubrick edited it again before the European release. I have only seen it once, but I enjoyed it. I'm a big fan of the movie.
The shorter cut was Kubrick's preferred cut. It's better paced and the hotel feels more isolated. He often recut films after seeing them with an audience, destroying the original version in the process. But because The Shining got a wide release in the US it was already in distribution by the time he recut it, so he made do with international viewers getting his final cut.