A server is a computer that supports multiple users. So whether it is your email server or your database server, your print server or your file server, these are assets that everyone in your company shares. When you log on to your office computer, you will see a C-drive for your internal hard disk and a D-drive for your CD-ROM. But in addition, you may notice the F-drive or G-drive where the bulk of your work files are found. Those drives are sitting out on another computer, most often somewhere else in the office. That computer is the Server.
The Server also allows a centralized location for management purposes. So the IT department can add users, change passwords, configure your security settings all from a central location. So that way regardless of what computer you log in to on the network, you can access your files, you can log in, you can get to the things you are allowed to see and run the applications you are allowed to run and that is all controlled through Server.
Servers can have a lot of issues with performance. They often become over-utilized, so they will be slow than they need to be maintained. Sometimes users may have trouble connecting to the server from the desktop and there can be a variety of issues around that. It could be the network or the server itself.
Licensing is sometimes an issue for servers because that is where the licenses usually are retained for a lot of applications, that way you can have multiple users all accessing the software at the same time, and it's the licensing that allows the server to keep track of who is using what applications.