TLA for Rigid Inflatable Boat. RIBs are the favoured transport of boarding parties of the Royal Navy and Royal Marines. The rigid bit is the hull and this enables the boat to go very fast over rough seas without having one's kidneys liquified and the blowy up bit provides the buoyancy. Sometimes referred to as Geminis, RIBs are great fun, unless you're being shot at... by the Iranians... for instance.
See here: Boats 'n' stuff