Simply put, a server is a very powerful computer, usually with redundant power supplies, hard disks, network cards, memory banks, etc. For small businesses the server is the central repository for documents. The latter simplifies access to documents, their management and perhaps most importantly their backup. Server can also have DHCP, DNS, WINS, DFS, e-mail, print, database and perhaps a web server role or in rare cases all of the above in one. In larger organizations, different roles are divided among physical or virtual servers as the load on each server is much greater.
We prefer IBM, Hewlett-Packard or Dell servers not only for their state-of-the-art performance, reliability and design, but also for their excellent post-sale support and online drivers and knowledgebase database.
As stated earlier, each company is unique and so are servers, each one tailored for specific tasks. It would be very hard to give you the best advice without a good understanding of what you would like to achieve with your investment.
