MEMS stands for MicroElectroMechanical Systems.
MEMS describes a technology where the tools and techniques typically used to create integrated circuits are instead used to create other devices. The techniques include photolithography, film deposition, and material etching. MEMS is a broad term covering devices with optical, fluidic, mechanical, and/or electrical features.
Integrated circuits are commonly made on silicon substrates making silicon the most common material for MEMS. MEMS is not limited to silicon based devices. MEMS are also made from compound semiconductors, polymers, and metals. Many MEMS devices do not include integrated electronics. Without electronics, a much larger set of materials can be used to create devices as contamination issues, such as heavy metals, that typically damage electronics are safely ignored.
A MEMS process can often be run in a fab currently creating integrated circuit devices by using a subset of the equipment and adding a few MEMS specific tools.