When building out your data center with servers, you really have 3 options. Either buy Tower based servers, rack servers or blade servers.
Deciding on which type of server farm you want all depends on the real estate you have to house them, your long term growth plan and of course your budget.
Tower servers are generally on the lower end price-wise but can grow quickly into a very expensive solution.
As I had mentioned above about real estate, tower servers take up a lot of space because they are individual systems that also require individual monitors, keyboards which would also require a place for these peripherals to set on.
Rack servers are commonly used in medium to larger sized data centers. These servers are normally mounted inside of a rack or medal cage that, for example, would be 42″ tall and 19″ wide.