Technically a server is a computer, but it is built for 24/7 operation: redundant power supplies, reliable disks and remote management. It doesn’t serve one person — the whole team uses it at the same time.
A server can be physical (in the office rack), virtual (several virtual servers on one physical machine) or in the cloud. The right option depends on your applications, the number of employees and the amount of data.
Example
An accounting database, shared folders and an internal business application on one office server — everyone works with the same data.