USB 2.0 and USB 3.0 are different speeds of USB; the connectors come in three sizes, but 2.0 and 3.0 are used with all three connectors. USB 3.0 is much faster than 2.0, but a 3.0 device will run slower at 2.0 speed if plugged into a 2.0 device. This is called "Backward Compatibility."
Micro USB refers to the USB connector size. The three sizes are referred to as Standard, Mini, and Micro. Micro USB connectors are the smallest, and they are used on many devices today, such as Galaxy phones, and Kindle readers.