Micro Servers are basically a NAS (Network Attached Storage device) with much higher processing power but with the Architechure seen in full Servers.
Micro Servers generally do NOT have enough power to run Windows Server software, but run Linux Server software beautifully. NAS drives also run a Flavour* of Linux as their operating system.
Linux (any Flavour* or Distro*) is FREE software Open Source which comes in both Work Station OR Server versions, 32 or 64 Bit versions, which requires much less memory and CPU overheads in order to work. Common Distros* of Linux are Ubuntu, CentOs, Mint, Red Hat and Debian, among others.
* Linux is a FREE operating system based on UNIX. Over years, variations on the Unix base have arisen, each with varying features and/or interfaces (such as GUI). These variations are called Flavours or Distro (short for Distribution).