A server is an end host that provides some service to clients (often regular PCs). Fundamentally, a service is just a computer running some sort of server software. In fact, it is possible for the same device to be both a server and a client.
Hardware
Servers can come in many shapes and sizes. Ultimately the ‘server’ is really just a software package that can (at least theoretically) be run on a wide array of computer hardware. A common PC tower, laptop, or even a cell phone can be a server.
That said, in the enterprise world servers are often rack-mounted devices, similar to enterprise routers & switches.
Hardware servers are available from a number of vendors, some of the most popular being Dell, HP, IBM, and many more. Cisco, for their part, offers their own line of server hardware, though they remain known mostly for their networking equipment. In many cases, servers can even be custom built with off-the-shelf or custom ordered components.