Cable Category

Category Maximum data rate Usual application :
CAT 1  Less than 1 Mbps  analog voice (POTS),Basic Rate Interface in ISDN
CAT 2  4 Mbps   Mainly used in the IBM Cabling System for token ring networks
CAT 3  16 Mbps   Voice and data on 10BASE-T Ethernet
CAT 4  20 Mbps   Used in 16 Mbps Token Ring
CAT 5  100 Mbps  100 Mbps TPDDI  1000 Mbps (4 pair)  155 Mbps ATM, Gigabit Ethernet
CAT 5E 100 Mbps   100 Mbps TPDDI, 155 Mbps ATM
CAT 6  200-250 MHz   Super-fast broadband applications

CAT 3 cabling is near-end crosstalk-certified for only a 16 MHz signal, while CAT 5 cable must pass a 100 MHz test.
http://infodeli.3com.com/infodeli/tools/hubs/suprstac/ss100/ssii100/appa.htm

The EIA/TIA defines the following five categories of twisted-pair cable:
Category 1-Traditional telephone cable.
Category 2-Cable certified for data transmissions up to 4 Mbps.
Category 3-Balanced 100  cable and associated connecting hardware whose transmission characteristics are specified up to 16 MHz. It is used by 10BASE-T and 100BASE-T4 installations. Category 3 is the most common type of previously installed cable found in corporate wiring schemes and normally contains four pairs of wire.
Category 4-Balanced 100  cable and associated connecting hardware whose transmission characteristics are specified up to 20 MHz. It is used by 10BASE-T and 100BASE-T4 installations. The cable normally has four pairs of wire. This grade of UTP is not common.
Category 5-Balanced 100  cable and associated connecting hardware whose transmission characteristics are specified up to 100 MHz. It is used by 10BASE-T, 100BASE-T4, and 100BASE-TX installations. The cable normally has four pairs of copper wire. Category 5 is the most popular cable used in new installations today.