What Are Muxes and Demuxes?

A huge part of FPGA design is using logic blocks in design. With logic blocks, you can compartmentalize your design, rather than trying implement everything in one shot. Designing without smaller blocks would be like trying to design a car without subsystems like the braking system or engine. About half of the way through the course there is a project that covers a variety of basic logic blocks, including multiplexers (muxes) and demultiplexers (demuxes). So what are muxes and demuxes?