SIPs are basically thick sheets of expanded polystyrene board sandwiched between two sheets of Oriented Strand Board (OSB) or plywood. They can be used to construct walls, roofs, floors — even basements. SIPs allow very little air to pass through them, so they create a super energy-efficient structure.
I'm not recommending one manufacturer over another, but Thermapan has several detailed videos illustrating SIPs and their advantages. For those with dial-up connections, they offer a free CD.