There are two main types of optical fibre: single-mode and multimode. A mode is a ray of light that enters the fibre at a particular angle. Multimode fibres use LEDs as the light-generating device, while single-mode fibres generally use lasers.
Multimode fibres allow multiple modes of light to propagate through the fibre. Because these modes of light enter the fibre at different angles, they will arrive at the end of the fibre at different times because they have travelled different distances. This characteristic is known as modal dispersion. Modal dispersion limits the bandwidth and distances over which multimode fibres can be used. For this reason, multimode fibres are generally used for connections within a building or a relatively small, geographically constrained area.
Single-mode fibres allow only one mode of light to propagate through them and as a result of this, modal dispersion is does not pose a problem. Thus, single-mode fibres are capable of delivering considerably higher performance connectivity over much larger distances, which is why they are generally used to connect buildings and over long distances.