A type of tire mainly used for racing. A tubular tire has no beads; instead, the two edges of the carcass are sewn together (hence the term "sew-up") with the inner tube inside. Tubulars fit only on special rims, where they are held on by cement.
Tubulars use Presta valves.
Most people find expensive tubulars too expensive for recreational riding...but cheap tubulars are distinctly inferior to good clinchers, particularly in that they tend to be lumpy and crooked.
Comparing high-quality tubulars with clinchers, including the rims, tubes, etc, tubulars save about 50 grams per wheel...but your bike winds up heavier, because you really need to carry a complete spare tubular, as opposed to a tube and/or a patch kit. This doesn't apply if the team car is carrying spare wheels/bikes for you.
If you don't glue your tubulars on properly, they can roll off, causing you to crash. If you get a flat on the road, you can't glue your spare securely, since the glue needs to dry overnight; as a result, you have to ride very gingerly on your spare, taking it really easy on the curves and descents. If you get two flats on the same ride, you're screwed.
Some people believe that tubulars corner better in the rain...but I never go fast on wet roads anyway. If you flat in the rain with tubulars, your ride is over, because there's no way to make a wet tubular stick to a wet rim.
Tubulars are fairly immune to "snake-bite" rim cuts, and may offer slightly better "suspension" action than comparable clinchers. Their rolling resistance is actually worse than with good clinchers in most cases, due to imperfect contact of the glued section with the rim, so there is rubbing.
Standard size tubular tires use a rim that corresponds in diameter to a 622 mm (700C) clincher rim.
Back in the 1970s, 622 mm clinchers were very rare in the U.S., and most sporty bikes used either 630 mm (27 inch) clinchers, or standard (622 mm) tubulars.
The fact that these sizes are so close led to an in-accurate habit of referring to "27 inch" tubulars. This careless nomenclature still causes confusion, and people often imagine that there is a different "27 inch" size in tubulars as there is in clinchers.
This is not true. THERE IS ACTUALLY NO SUCH THING AS A "27 INCH" TUBULAR.
All full-size tubulars fit all full-size tubular rims.
There are smaller tubulars, "26 inch", "24 inch" and even smaller, but those are VERY uncommon, mainly used for children's race bikes, which hardly exist at all in the U.S. The smaller sizes are very poorly standardized; tires and rims of nominally the same size may not fit one another.
There are several articles about tubulars in Jobst Brandt's FAQ section: