The Arduino Micro packs all of the power of the Arduino Leonardo in a 1.9" x 0.7" (48mm x 18mm) size. Although it may look like a stick of gum, it is actually a USB-native 8-bit microcontroller, with 32K of flash, and 2.5K of RAM. You can program it directly via the USB micro connector on one end (or for the advanced users, the 6-pin ISP header). The Arduino Micro is a microcontroller board based on the ATmega32u4.
Onboard is the processor, crystal, micro USB connection with fuse, Reset button, ISP low-level reprogramming header, ON LED, TX & RS LEDs, a extra user pin #13 LED, and a 3.3V regulator. You'll probably also want to pick up a micro-B USB cable to connect this board to your computer.