A nanobot is a very simple microscopic robot that is small enough to interact with matter on an atomic level. Nanobots are easily manufactured (specifically, by other nanobots), and employed in a wide variety of applications.
A single nanobot is utterly useless: its programming is too simple to achieve any meaningful task, its power storage is too limited for it to operate for very long, and its toolset only allows it to manipulate individual molecules or very small amounts of energy. However, when you get several billion nanobots working together, there is almost no limit to what they can accomplish.
A nanobot colony runs as a highly redundant distributed computing network, with each individual nanobot serving as a node with a communications range of less than one millimeter. The colony's overall program runs on this network, running the nanobots in concert to achieve its goals, whatever they may be.
One of the most common uses of nanobots is in healthcare, because nanobots are an ideal supplement for a living body's natural defenses. They can identify and eliminate pathogens and foreign organisms, destroy or repair tissues, monitor and regulate chemical levels, and anything else that takes place at the molecular level in the body.
Medical nanobots come in two basic varieties: autonomous and subordinate.
Autonomous medical nanobots (AMNBs) are given a fixed program and then injected into the patient's body to carry that program out. Most often they are used either to combat disease by attacking foreign microbes, or they are used to look for traumatized cells or tissues and repair them.
Subordinate medical nanobots (SMNBs) usually have some basic programming, but do nothing in the body until instructed to by an outside source, typically a computer that is monitoring the patient's condition, and which in turn is usually receiving instructions from a physician or surgeon. This allows the medical professional to essentially perform microscopic surgery on the patient without ever entering their body.
Nanobot manufacturing methods have several major advantages over traditional manufacturing. First, there is the fact that the manufacturing process is always the same: whether you are making computer chips, articles of clothing or weapons of mass destruction, your manufacturing nanobots simply assemble the product atom by atom. All you need to do is make sure the nanobots have an ample supply of free atoms to work with, and an accurate molecular pattern to follow. Another advantage is the expense: once you have your manufacturing nanobot colony up and running, all you need to do is supply energy, raw materials and instructions. The equipment you need to maintain is generally simple and inexpensive, and the nanobots can generally maintain themselves, so your costs are minimized.
The main downside to nanobot manufacturing is time. Although nanobot manufacturing products can be highly complex and use materials that are difficult or impossible to work with by traditional methods, because the assembly occurs atom by atom, the work tends to go slowly. For this reason, nanobots are most often used in manufacturing very small items that would be difficult to build working on a macroscopic scale, such as computer chips or small machine parts. Nanobots are also commonly used in manufacturing when the purity of the material being produced is crucial, such as matter/antimatter reactor parts.
The temptation to use nanobots as a form of weapon has always been great, but they also have limitations.