A USB is a Universal Serial Bus. It is a device that can "allow many peripherals to be connected using a single standardized interface socket and to improve plug and play capabilities by allowing hot swapping; that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device." USB drives, chargers, and even coffee mug warmers can be attached to USB ports.
Today, many of us carry around a USB drive, a backpack, or both, so why not merge the two? After all, even a small memory chip no larger than your thumbnail can hold 16GB of memory. Think about how much you could hold with a hard drive embedded somewhere in your back pack? We are talking at least 1 terabyte of storage. The backpack would plug in to a computer's USB port via an extendable/retractable cable connecting the built-in hard drive to the port. Then, designers could make it such that the hard drive can be de-attached from the backpack.
In the future
Someone could modify a human to have USB ports near their brain. Then, they could upload and download information from any flash drive or computer!
What would happen if...
If your computer does not have enough USB ports, you can ususally get a USB hub. These devices connect to a single USB port, but have many USB ports on them, sometimes up to eight. In theory, you could just keep connecting USB hubs to the computer, then connecting USB hubs to USB hubs and on and on. Then, you'll have like one million ports for one million plug-and play devices!
Let's say you have three USB ports on your computer, and a typical USB hub contains 4 ports. If you plug in all three USB ports with these hubs, then you have twelve open hubs. If you plug in hubs to each of these twelve ports, you have 48. The numbers increase exponentially: 192, 768, 3072, 12288, 49152, 196608, 786432, 3145728, 12582912, 50331648, 201326592, 805306368, and that's when my phone calculator phases out. However, this would cost tons of money, so don't attempt unless you're rich!