Servers are larger machines than desktop PCs in terms of CPU, memory and disk drive space. Desktop PCs are designed to be used by one person at a time although they are running multiple applications quite often.
Servers can be used in several ways. One is as a file server, another is as a security server, and they could be an application server. There are other ways they could be a server on a network.
A file server is a centralized computer where we keep all the files that are used by multiple people on multiple computers. By using a server to keep the files on, people will always be accessing the latest copy of a file. Not using a server, you must keep the file on multiple machines and never know if it is the latest revisions.
A security server can be used to keep up with all the users and their passwords on your network. Instead of lots of copies of this information on lots of machines, making changes difficult, when someone logs in their computer on the network, their computer contacts the server to allow the user to login or to reject them for security reasons.
An application server is one that we keep the applications (or called apps often today) or programs on and the you only need to buy and install one copy of the application on the server instead of on all the PCs. However, you will still need to have licenses for all the users, sometimes based on maximum number of users using at one time or others by max number of machines that will use ever. This in turn makes sharing files also run smoother.
An advantage of a server is you only have to put the larger memory and disk space on one machine, not many. You will need more space both ways on the server as more than one machine is sharing it. You also need a server operating system instead of a PC operating system so it knows how to work with multiple machines at one time. Popular server operating systems include Windows Server and Linux and UNIX.
Thanks, Chip, for the question.