Fast Ethernet is a generic term for a family of high-speed Local Area Network (LAN) types running at 100 Mbps over either UTP or fiber-optic cable. These networks use the same Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol that "normal" 10 Mbps Ethernet uses. In fact, the main difference between Fast Ethernet and the older 10 Mbps system is speed.
Fast Ethernet is currently the upgrade path most users of 10 Mbps Ethernet systems are using to speed up their networks. There are several reasons for this popularity. The first is that it is fairly easy and inexpensive to convert to Fast Ethernet as opposed to FDDI. Fast Ethernet's support for all Ethernet frame types and software ensures that every machine upgraded does not need a lengthy reconfiguration to make it work after the upgrade. Normally, all that needs to be done to a PC to upgrade it to Fast Ethernet is to replace the Network Interface Card (NIC) with a new Fast Ethernet card and load the drivers for the new card.
In the real world, however, things are not always as easy as we would like them to be. For example, many older networks are built with coaxial cable. This cable type is not supported by Fast Ethernet, so it has to be replaced. Also, Fast Ethernet has some very stringent distance limitations which need to be accounted for. See the Design Guidelines page for more details on how to plan a Fast Ethernet LAN.