This is actually a good question considering most users are used to devices being rated by MB capacity, such as data storage solutions and RAM. This becomes confusing when shopping for equipment such as network devices and certain aspects of motherboard to device communication where the hardware is rated by its Mb/sec capacity. One thing to remember is that Mbs converted to MBs is not equal to an actual MB. For instance, LANs operate at(usually) 100Mb/sec. Your LAN connection can transfer 100,000,000 bits/sec at that speed(1Mb=1,000,000bits). Take 100,000,000 divided by 8(the amount of bits in a byte) and you get 12,500,000 bytes. Take that and divide by 1024(the multiplier for data storage conversions) and you get 12208 kilobytes. Do it again and you get 12 megabytes. So your LAN connection speed is only transferring 12MB/sec of information. Bits per second is used to reference transfer speeds(since data is sent a bit at a time) while bytes are used for data storage capacities(since data is stored by byte not bit, if I remember correctly).