A RIB is a Rigid Inflatable Boat. It has a solid, usually v-shaped hull and inflatable tubes for gunwales.
The solid hull gives it the handling and performance to be used safely in rough conditions, and the soft tubes provide a gentle cushion to soften low-speed collisions. It is a development of the inflatable boat allowing for much greater loads and more stability. Most RIBs have a hull designed for hydroplaning at a reasonably low speed, minimising drag. The naturally buoyant nature of the inflatable tubes enables the boat to be completely filled up with water without sinking.
RIBs are commonly used in yachting as tenders to larger boats, safety craft in sailing, or as (professional) rescue boats. Most are outfitted with one or two outboard engines, but larger ones may be outfitted with inboard diesels. The smaller ones are usually very easy to drive, although a certain level of competence can make the ride a lot more comfortable and in rough weather even a RIB can't keep an unskilled captain absolutely safe.
RIBs are sometimes referred to as Zodiacs after the French company that developed and marketed the first succesful RIB. Another nickname is rubber, since many people inaccurately perceive the tubes to be made from rubber.