Quite simply, to conduct any form of communication or activity online; be it e-mail, search or web browsing, shopping & file transfers to simply buying songs on iTunes, somebody somewhere has to host and manage the equipment on which the service runs in order for the service to work.
The size and type of business you run, dictates the size and type of hosting service the business will require. There are a few basic types of hosting.
All OS flavours supported and catered for.