"Megabits per second": 106 bit*second-1, a bandwidth measurement.
This unit is usually used for network speed measurements (both physical layer or throughput). For example, Ethernet comes in 10Mbps, 100Mbps, 1Gbps, and now 10Gbps speeds. Cable modem and DSL are just barely into the Mbps range.
Mbps is also commonly used for measuring MPEG2 streaming video; for example, digital cable often uses a 3.75Mbps stream per TV channel and runs over multiple 35-38Mbps QAM channels.
Bus measurements (like PCI or IDE speeds) typically use Megabytes per second (MBps, capital "B") instead. 64 bit PCI at 66 MHz is 528MBps, or 4.224Gbps.