Servers may use many different operating systems, but the ones you will most often encounter will be using either Unix or Linux.
What is an operating system?
There are more than a few. What others are out there? (note: this page is dated, but it is a good historical comparison) Evolution of Operating Systems (also somewhat dated, but also useful for the historical background)How Operating Systems Work - to quote How Things Work,
At the simplest level, an operating system does two things:
- It manages the hardware and software resources of the computer system. These resources include such things as the processor, memory, disk space, etc.
- It provides a stable, consistent way for applications to deal with the hardware without having to know all the details of the hardware.