Tubes are what keep your tires full of air: they are the inflatable innards between the wheel and the tire. They come in several varieties, and it’s important to know which one works for you, not just in terms of size and valve but in terms of type.
Like tires, there are two common sizes for tubes: 26″ and 700c. In terms of width, it will correspond to the tire, though there is a range in tube sizes, as inflatable rubber is a little forgiving.
The difference between valves is one not encountered in tires. Valves are the mechanism by which the tubes are inflated. There are two types of valves; Presta and Schraeder. Schraeder valves are wider at the base, and will not fit on a rim that has a hole shaped for a Presta valve. On the other hand, Presta valves will fit rims with holes for Schraeder valves as long as an washer holds it secure. Air compressors, such as those available at gas stations and many bike stores, are compatible with Schraeder valves. A screw-on adapter, though, can make Presta valves compatible. Many newer pumps will have smart heads, meaning they will work on either Presta or Schraeder valves indiscriminately.
Some tubes come equipped with extra long valves. Those are to accommodate deep dish rims. Deep dish rims serve a couple of purposes: they are more aerodynamic, for that extra edge towards going faster, and on mountain bikes they provide a little more up-and-down stability. They’re uncommon except where speed performance is a major concern. Long-valve tubes are unnecessary except with deep dish rims.
A regular tube is suitable for most riding.
Thorn tubes, which are puncture resistant and over four times the thickness of a regular tube, offer significantly more flat protection when riding over routes such as the Galloping Goose, which can have thorns, bits of wire and glass, and small pieces of metal that will puncture a regular tube. They also offer fewer pinch-flats, as it’s harder to pinch the thicker rubber.
The one drawback of thorn tubes is the additional rotating weight: it does add a few grams. For almost all riders, going a hair slower for the same effort is more than made up for by saved time repairing flats.
Downhill tubes, commonly referred to as DH tubes, are even more heavy duty, designed for serious downhill riding. Thicker than regular tubes, they are also wider in circumference, designed to not expand as they are inflated. Not expanding means the rubber doesn’t thin at all, which makes pinch flats more unlikely, and also means that a punctured DH tube will deflate and not pop like a regular tube can in some circumstances.
DH thorn tubes are also available, even thicker and more puncture resistant. On the far opposite end of the spectrum from DH thorn tubes are ultralight tubes. They cut weight fractionally when speed performance is the biggest goal, but also go flat more easily. If speed is the primary objective, it might give bigger returns to look at upgrading the tires.
Slime is sometimes brought up as an answer to flats. A green goo injected into your tubes, Slime is designed to plug punctures as they happen and then dry in place. The problem with Slime is that, when it fails, it gets green goo all over everything and makes it tough to change the tube.
There are alternatives that don’t involve tubes at all: tubular and tubeless tires. Tubular tires go on high-performance road racing bikes, and are tires and tubes in one. They fit on rims made specifically for them, and are glued in place. Extremely difficult and expensive to replace when they go flat, tubulars aren’t the ideal choice for most riders.
Tubeless tires allow the use of lower air pressure for better traction without the possibility of pinch flats, as there is no tube to be pinched between the rim and an obstacle. The rim must be airtight, though, and a rider has to carry a spare tube in case they get any kind of puncture. Tubeless tires are also extremely difficult to seat on the tire.