Multi mode and single mode are two types of fiber optic cables. Multi mode fiber has a larger core diameter than single mode fiber, allowing multiple light rays to be transmitted at the same time. This makes it ideal for short distance communication, such as within a building or campus. Single mode fiber has a much smaller core diameter, allowing only one light ray to be transmitted at a time. This makes it ideal for long distance communication, such as between cities. Both types of fiber optic cables are used in a variety of applications, from telecommunications to data centers.