As above really.
Ta
Tubular Tyres...
Ie they are glued onto the wheel, not clincher like a MTB tyre, or normal roaad wheel.
tubular tyres.
http://www.bikeradar.com/road/gear/article/workshop-gluing-tubular-tyres-for-cyclo-cross-28099
The Tub (tubular tyre) is completely circular in cross section. There is still an inner tube to hold air and then the tyre is sewn up on the underside (rim seat face). The tubs are then glued on to the rim with either a specific double sided tape or liquid cement. It's a bit of an art to get right and some glues can soften causing a tyre to roll off in high temps/heavy breaking. At least at Tour riding level.
They are supposed to be far more supple and have better rolling character than clinchers and as there is no rim walls pinch flats are VERY hard to induce. I think at the Pro level almost everyone still uses tubs. Possibly, certainly in TTs.
And if you think MTB tyres are expensive, the price of a good tub will make you sob in your beer.