The Server's Various Roles
Servers do, essentially, what their name implies: They provide services. These services come in lots of shapes and colors, but typically involve mass file storage or large amounts of processing. A server can also act as a sort of PC but for multiple people at a time. You see, servers and their software are configured so that multiple users can be logged into them at any given time, and the server handles all of the requests for information or processing, simultaneously. So you can see why your average PC system probably wouldn't be up to the task of being a real server.
That isn't to say though that all servers have to be really high end expensive pieces of machinery designed to keep the world running, or anything like that. You can, technically, run a "server" on very outdated equipment, perhaps an old PC that you don't use anymore, that you have configured to, for example, store files that can be accessed via FTP over the web. So long as not very many people are trying to use it at the same time, that old equipment will be plenty sufficient for that meager usage.
Most true servers though, have a lot more than just that on their plate. In addition to storing massive amounts of information, while they're doing that they could be hosting websites with 1,000 users browsing them, as well as acting as a firewall and router for an internal network and processing commands for 3 different administrators logged in via workstations. The life of a server is tough - far tougher than a PC could ever handle. That is why servers have to use special hardware and technologies to keep everything going.