A Windows server operating system can be used for many different things where as a non server version often called a workstation version is mainly used as a client that connects to the server. Servers are often used as a centralized location to store files and other data such as websites, applications and databases. Then all of the clients (workstations) can connect to this location over the network to get the information they need.
One of the main things Windows servers are used for are domain controllers. Domain controllers act as a central place where users log into the domain for authentication. A domain is a group of computers and devices on a network that are administered as a unit with common rules and procedures. Depending on their credentials the clients will be able to access resources on the network that they have permission for after they log into the domain.
There are more versions of standard/client Windows operating systems compared to Windows server operating systems. More recent client versions include 7, 8 and 10 where server versions include Windows 2008, 2012 and 2016. Home users generally use one of the standard versions. In a work environment you will have more workstation computers than server computers. One server computer can provide resources to hundreds or even thousands of clients depending on the configuration.