Hose fittings come in a large range functions and shapes.  Essentially, a fitting can be any component in a hose system, which is added to the main hose to adapt it to a particular function.  The basic role of the main hose is simply to transfer fluids or liquids between two points, but a fitting is required whenever there is a transition between the hose and a component, or even between two hoses.  Fittings are usually constructed of more rigid material than the hose, most typically plastic or metal.  There is a mechanism for them also to be attached to the hose in some manner that provides a seal if they are part of the fluid transfer. 

These components serve many functions.  Many times a hose enters a network that has a smaller form of the hose and needs to be adapted to that network; fittings are used to facilitate this conversion.  In other cases, there are places in a hose routing, which even the flexibility of the hose cannot provide a good fit, and angled fittings are used to route the hose in the proper direction.  One of the simplest fittings is an adapter, which connects hoses of the same form and size, used to provide a leak repair or extension.