A conduit is simply a pipe through which things can be passed. In manufacturing and industry, the term is most commonly used to refer to conduits used in wiring. This type of conduit is used to protect, insulation, and isolate wiring such as cables, electrical wiring, and so forth. There are a number of different types available, and in some settings, the building code may require the use of a specific style of conduit for health and safety reasons.
The origins of conduit use lie in the early days of electrification. As people ordered electrification of their homes, electricians commonly used the pipes which had been used to carry gas to run electrical wires in a way which would be unobtrusive and handy, relying on existing systems to run a network of wiring, rather than cutting holes to set up a new system. Over time, electricians realized that in addition to being convenient, the pipes actually served several important functions, and pipes began to be installed with new electrical systems.
One advantage of running wire through a conduit is that it is protected from the elements. When wiring runs outside, through a parking garage, through a lab, or in other areas where it might be exposed to water, chemical vapors, and so forth, running it inside conduit can protect the wire from degradation. This means that it will not need to be replaced as frequently as exposed wiring, and it will reduce the risk of electrical shorts which could lead to fires, equipment failures, and other problems.
Using protection for cabling and electrical wiring also isolates it from impacts. If wiring is located in a high traffic area, near an elevator, or so forth, it may be protected with conduit so that the conduit will take the brunt of any impact or stress. This can also be important when wiring is in an area subjected to stress.
Both rigid and flexible styles are available. Rigid conduit can offer more protection, but it is harder to work with, because it must either be bent or coupled together as it is brought through a structure. Flexible versions are easy to install, but may confer less protection on the enclosed wiring, although they give instead of shearing or bending when subjected to stress, which can be useful. Metallic and nonmetallic options can be found at many hardware stores and stores which cater to electricians. It's a good idea to read the building code carefully to check for any restrictions and requirements before embarking on a wiring project, to confirm that the right materials are used.