An internally-geared hub is a gearing system contained within the rear-wheel hub of a bicycle. All the components are contained within the sealed hub, making them less susceptible to the elements when riding. They are generally regarded as lower maintenance than dérailleur gears. Usually they will have a somewhat narrower range of gears, and larger steps between the gear ratios.
Some IGH may really be a [CVT] internally - if there is a smooth ramp from hard to easy, with no discreet gears then that's a good sign it's something other than an IGH.