It is a strange term, but basically it is a tire that mounts like a clincher, but is made like a tubular that isn't sewn shut. Instead of being one piece of poured rubber like a regular clincher tire, open tubulars are handmade and the riding surface is glued on to the sidewalls. Let's see... Here is a good explanation and picture: Technology

The only problem is that they are notoriously hard to get on the first time. They aren't even rounded like a clincher and the tread portion is totally flat. On my last set I managed to get one on by hand, but for the other I cheated and used the VAR tool. After riding for a while they stretch out and are easier to remount (in case of flat on the road).