Changing Temperature, Changing Density. Most matter increases in volume when it gets hotter. For example, if an iron rod is heated, it will get longer and fatter and its density will decrease. This happens because the mass of the rod stays the same, but its volume increases. The increase in the volume of matter with increasing temperature is called expansion. When cooled down, most matter decreases in volume and increases in density. This decrease in volume is called contraction.
USB-C also builds on the new USB 3.1 standard so to all intents and purposes is the connection type that brings in the new power and speed advantages of USB 3.1. USB-C is still backwards compatible with existing USB variants, but that of course requires adapters.
Now that there are many cheap options for USB 3.0 flash drives, there really is no reason to get a USB 2.0 drive. USB 3.0 brings much better file transfer speeds than what USB 2.0 could deliver. For example, the best USB 2.0 flash drives could only transfer files at speeds of 30 to 40MBps. The best USB 3.0 flash drives are closing in on 250MBps. It's important to note that USB 3.0 drives work in USB 2.0 ports; you'll just be limited to USB 2.0 speeds.