Shown above, top-to-bottom: loose-bearing bottom bracket with square taper spindle, square taper cartridge bottom bracket, Shimano Octolink bottom bracket, Truvativ external bottom bracket cups.
Other thread types, mostly found on older frames, are Italian, French, Swiss, as well as some obscure others. The thread types differ in the size of the threads, and the direction of the threads on the drive side and non-drive side.
Many older bottom brackets use loose ball bearings, which are held in place with retainer rings. A bottom bracket overhaul is the process of disassembling the bottom bracket, cleaning all of the parts, cleaning the threads in the bottom bracket shell, cleaning the bearings (or replacing them, if necessary), then re-assembling the whole system. When assembled, the cups that hold the bearings in place must be tightened just right--tight enough, but not too tight--to provide the smoothest possible pedaling action.
As with most other bike components, a liberal amount of grease is spread on the threads before installing them. This prevents the threads from slipping out of adjustment, and prevents them from rusting and freezing up, making adjustments and removal easier in the future. Sometimes, the bottom bracket threads are also wrapped with teflon plumber's tape. This helps to create an even sturdier bond, as well as helps to eliminate knocking and grinding noises coming from your pedaling motion.
Most modern bottom brackets use cartridge bearing systems, which are usually a self-contained unit. Some cartridge bearings are sealed to help protect against water and other contaminants. These bottom brackets provide a less maintenance-intensive system, with a much longer lifespan. The overhaul process with this type of bottom bracket is somewhat simpler; sometimes the sealed bearings can be replaced, but usually the whole bottom bracket is replaced.
The primary distinguishing feature among most modern bottom brackets is the method by which the crank arms are attached to the spindle. Historically, as well as currently in many low-end to medium-quality bikes, the most common spindle is the square taper. On a square-taper bottom bracket, the ends of the spindle are shaped like a square with the corners somewhat rounded off, but the size of the square gets bigger as you move inward along the length of the spindle. The mounting holes for the crank arms have a hole with a similar square shape. The crank arms are simply pressed onto the spindle and held in place with a bolt; as the bolt is tightened, the crank arm gets tightened into place as the bolt wedges it further and further onto the thicker part of the spindle.
The advantages of the square taper system are that it is common, and therefore easy to find replacement parts, and relatively simple. The disadvantage to this system is that because the crank arms get wedged tightly onto the spindle, removing them for service can be difficult. After you remove the bolt, you must use a special crank arm remover tool to un-wedge the arm from the spindle. After this process is done over and over through the years, the mounting holes in the crank arms can get worn out, to the point that they can't be tightened enough to stay in place reliably on the spindle, so the crank arms must be replaced.
You should note that not all square-taper parts are interchangable, as there are some differences in spindle size and taper size among different manufacturers (e.g. Shimano vs. Campagnolo).
To improve on some of the deficiencies of the square taper design, Shimano designed a system that they called Octolink, where the ends of the spindle have 8 splines, and the inner edge of the crank arms have 8 corresponding grooves. There are two versions of Octolink, referred to as V1 and V2, which differed in the depth of the grooves between the splines. Shimano patented this design, and so not very many other manufacturers made parts that were interchangable with Shimano's system, as other manufacturers had to pay a royalty to Shimano to use their design.
To get around the Shimano patent issue, King Cycle Group, Race Face, and Truvativ got together and came up with their own splined bottom bracket design, and published the specifications as an open standard, so that anybody could produce parts that were interoperable with their design. They called their system the International Splined Interface Standard, or ISIS for short. ISIS uses a set of 10 splines on each end of the bottom bracket spindle.
Both Octolink and ISIS were popular with racers and other riders requiring high performance characteristics, as the increased diameter of the bottom bracket spindle provided a much stiffer, and therefore more efficient, interface. However, with the ISIS design, the increased diameter of the spindle required that the individual bearings be much smaller in order to fit them inside the bottom bracket shell. Because of the smaller bearings, the ISIS bottom brackets are prone to wear out much faster than the other designs.
To provide the stiffness advantages of Octolink and ISIS bottom brackets, and overcome the durability issues of ISIS, most major component manufacturers have come up with their own designs using external bearing bottom brackets. These designs have seemed to be very successful, and are now very common on many mid-range to high-end bicycles. They can be recognized by the knurled bearing compartments visible between the crank arms and bottom bracket shells. These systems are very easy to install, with only one special tool required to tighten or loosen the bottom brackets cups into the shell, and no special tools required for crank arm installation or removal.
Shown above: external bottom bracket as visible on the Raleigh Clubman bike.