Once you've decided to buy a server, your next step is to decide what kind to buy: a tower, rack-mounted or blade server. The size of your organization and the cost of the server will likely factor in this decision.
Tower server: The most basic servers are tower servers, which look a lot like workstation-class PCs. Tower servers are designed to sit on a desk or on the floor and nearly always have feet, a surprisingly important element because tower servers can get heavy and need a little help to remain stable.
Weight and stability become an issue as servers grow, which they need to do in order to house more hard disks and other components needed to cope with more demanding applications. Some tower servers can therefore be mounted inside standard 19 inch racking, at which point they start to compete with another type of server: rack-mounted servers.
Rack-mounted server: This type of server looks a little like a pizza box and will, for entry-level servers, be just 1.75 inches high. That's because racks are measured in standard increments called "units" and one unit is 1.75 inches high. You'll see rack-mounted servers described in terms of how many units they occupy, with a one-unit tall server described as "1U", a two-unit device "2U" and so on.
1U servers are nicely thin and can be very capable devices, but the design compromises because it squeezes all those components into such a slim chassis, that they are not easily expandable. Rack-mounted servers get around that issue by getting larger: some reach 4U, giving them the space to pack in much more equipment to create a more powerful server.
Blade servers: The smallest servers of all are blade servers. To understand blades, you need to know that these servers must be slotted into a dedicated chassis. The chassis contains slots that blade servers slide into and also does a lot of work like mediating input and output between the blades it houses and the rest of the network.
Blade servers are designed to be easily replaced--taking one in and out of a chassis is not much harder than assembling Lego--and are even more densely-packed than rack-mounted servers. The upside of this arrangement is that blades make it possible to put a lot of servers into a very small space.
Blade chassis can be as little as 4U or 5U tall, yet they can contain 10 or more servers. Compared to a single 4U rack-mounted server or a tower server, blades therefore offer exceptional density.
Your first server will almost certainly be a tower: the cheapest and simplest form of server. 1U rack-mounted servers are very competent devices but they lack expansion options that smaller businesses will find very useful in the future. Tower servers are therefore the most likely entry-level server for the majority of small businesses.
The limited ability of rack-mounted servers to take on new drives and memory make them better suited to larger businesses that know exactly what the server will be doing and will tailor it to that need.
Blade servers are generally for big businesses, yet small businesses may also find them attractive because blades are fabulously expandable.
Combined with the fact that the blade chassis can also hold storage and networking equipment, blades offer a small business that is willing to make an admittedly large initial investment several years of expansion options.
Investing in blades will, however, require at least $10,000 on the chassis, the servers, storage and ancillary networking equipment. Rack-mounted servers require the additional purchase of racking, which is generally well under $1,000 but is bulky and unsightly.