A clutch kit is a group of manual transmission parts that are either purchased as a unit or assembled piecemeal. When bought together, a clutch kit will usually consist of a pressure plate, clutch disc, and a release bearing. In certain applications a pilot bearing will be included, and many will also come with an alignment tool of some kind. Some manual transmissions are designed so that the release bearing is an integral part of the slave cylinder, in which case this will often be included as well. Most clutch kits are designed to the same specifications as the original parts, though it is sometimes possible to find a performance or racing kit.
The purpose of a clutch kit is to provide all the necessary parts to repair the clutch on a manual transmission vehicle. Clutch discs are wear items that will eventually need to be replaced, regardless of how carefully a vehicle is driven. While other parts, like the release bearing and pressure plate, may not wear out at the same speed, they may be replaced anyway since removing a transmission is often labor intensive.