Typically, the bolt consists of a tube of metal inside of which the firing mechanism is housed, and which has at the front or rear of the tube several metal knobs, or "lugs", which serve to lock the bolt in place when rotated by the operator.
A loose definition of what is encompassed by bolt-action firearms might include straight-pull designs that use a rotating bolt, such as the German Blaser R93 rifle, but most commonly it is restricted to manual turn-bolt designs.
By the 1890s, every modern nation had adopted a bolt-action rifle, though the United States was one of the last, with the Krag-Jorgensen. The designs of Peter Paul Mauser were perfected in 1898, and nearly every modern bolt-action rifle since then is a derivative of his work, changing very little.
Most bolt-action firearms are fed by internal magazines, though some sport removable versions of the same. Typically, the capacity is around 5 rounds, as this permits the magazine to be completely flush with the bottom of the rifle, thus preserving the aesthetics and clean lines of the firearm.
After World War II, the military bolt-action rifle was superseded with the semi-automatic rifle and then the assault rifle for regular combat usage. However, the bolt-action variety is still a standard weapon for sniper duty.
See Also:
Krag-Jorgensen, Lee-Enfield, Mannlicher-Carcano, Mauser, Mosin-Nagant.