Micro-Electro-Mechanical Systems (MEMS) are tiny machines that integrate "mechanical elements, sensors, actuators, and electronics…through the utilization of microfabrication technology." This site provides a good introduction to MEMS, how they are built, and how they operate. The most descriptive section is The Beginner’s Guide to MEMS Processing, which explains the processes of deposition, lithography, and etching. For each of these, there are illustrations that show what happens at the microscopic level. Other sections tell the history and current direction of research and development, as well as the challenges that face this growing technology.