A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are chrome plated or stainless steel to prevent rust or simply for aesthetics.
Before the safety bicycle, bicycles did not have chains and the pedals were attached directly to the drive-wheel, thus limiting top speed by the diameter of the wheel and resulting in designs with front wheels as large as possible. Using chain drive allowed the mechanical advantage between the drive and driven sprockets to determine the maximum speed, thereby enabling manufacturers to reduce the size of the driving wheel for safety. It also allowed for the development of variable gearing, allowing cyclists to adjust their gearing to the difficulty of the terrain, on the fly.
How to best lubrication a bicycle Chain is a commonly debated among cyclists. Liquid lubricants penetrate to the inside of the links and are not easily displaced, but quickly attract dirt. "Dry" lubricants, often containing wax or Teflon, are transported by an evaporating solvent, and stay cleaner in use. The cardinal rule for long chain life is never to lubricate a dirty chain, as this washes abrasive particles into the rollers. Chains should be cleaned before lubrication. The chain should be wiped dry after the lubricant has had enough time to penetrate the links. An alternative approach is to change the (relatively cheap) chain very frequently; then proper care is less important. Some utility bicycles have fully-enclosing chain guards which virtually eliminate chain wear and maintenance. On recumbent bicycles the chain is often run through tubes to prevent it from picking up dirt, and to keep the cyclists leg free from oil and dirt.
Twenty half-links in a new chain measure 10" (254 mm), and replacement is recommended before the old chain measures 256 mm (0.7% wear). A safer time to replace a chain is when 24 half-links in the old chain measure 121/16 inches (0.4% wear). If the chain has worn beyond this limit, the rear sprockets are also likely to wear, in extreme cases followed by the front chainrings. Replacing worn sprocket cassettes and chainrings after missing the chain replacement window is much more expensive.
The Wikibook "Bicycle Maintenance and Repair" explains that the difference between derailleur chains commonly labelled 8-speed, 9-speed, and 10-speed is in its external width (all are 3/32" chains).
In the case of derailleur gears the chain is usually long enough so that it can be shifted into the biggest front chain ring and the biggest rear cog without jamming, and not so long that, when shifted into the smallest front chain ring and the smallest rear cog, the rear derailleur cannot take up all the slack. Meeting both these requirements is not always possible.
In the case of single-speed bicycle and hub gears, the chain length must match the distance between crank and rear hub and the sizes of the front chain ring and rear cog. These bikes usually have some mechanism for small adjustments such as horizontal dropouts, track ends, or an eccentric mechanism in the rear hub or the bottom bracket. In extreme cases, a chain half-link may be necessary.