Micro-Electro-Mechanical Systems (MEMS) as the name describes are small mechanical components mass produced on silicon wafers, using fabrication techniques developed for Integrated Circuits. They are used in a range of devices from pressure transducers to small motors. In addition new classes of components have been produced using MEMS technology to produces optical switches using small mirrors built on top of CMOS device wafers.
Due to their small size, MEMS devices have become practical in applications such as tire pressure sensors. Cell phones and hearing aids all use MEMS microphones, partially due to size, and also because mass production manufacturing techniques have made MEMS devices much cost effective to use.