Nuke Luper and Chairboy are right. If you open your computer case, you'll find that the cable that attaches to the floppy drive has another connector on it. That's for the B drive.
Back in the good old days, Hard Drives were expensive. The original 8086 PCs very often came equipped with two 5 1/4" inch floppy drives (and those 5 1/4" were sierra-hotel compared to the 8" models). These things were really floppy. Not only was the magnetic media flaccid, they were encased in some sort of cardboard. Write protection usually involved a hulepunch. IIRC, at Single Density, a single DOS 5 1/4" floppy side at single density could hold 160 kb. Of course, you could flip it over. As time went on, you got drives that could read both sides, and you got "double density" disks, bringing it up to the maximum of 640 kb.
When the first mac was released, there hot media on the market were these smaller floppies encased in plastic. The two big competing formats were 3" and 3 1/2". The 3 1/2" won out, and became the standard on Macs, Amigas, and atari sts. Maximum storage was around 720 kb ("double density, double sided")
On the PC front, the 3 1/2 became standard much later, probably around the late 80s. Up until the first pentiums, it was standard to see a PC equipped with both a 5 1/4" and a 3 1/2" drive. And the B drive would often as not be the 3 1/2".
