A micro server is a type of server that is designed to be used for small- to medium-sized business/computing application and services. It is one of the form factors of a standard server but with much less processing power, storage capacity and other peripheral support.
A micro server is also known as a server appliance.
Techopedia explains Micro Server
Micro servers have less computational power, and fewer operational and maintenance requirements than standard servers. Typically, they are designed for a specific business/computing use and come preinstalled with a suite of purpose-specific applications, such as to be used as an email server, VPN or as a small firewall. Generally, a micro server consists of between one and two processors, and up to four slots for memory (RAM). It can also support up to four disk drives and will have multiple Ethernet, USB and other external connectivity options.