At first, miners used their central processing unit (CPU) to mine, but soon this wasn't fast enough and it bogged down the system resources of the host computer. Miners quickly moved on to using the graphical processing unit (GPU) in computer graphics cards because they were able to hash data 50 to 100 times faster and consumed much less power per unit of work. During the winter of 2011, a new industry sprang up with custom equipment that pushed the performance standards even higher. The first wave of these specialty bitcoin mining devices were produced by Butterfly Labs. These easy to use miners were based on field-programmable gate array (FPGA) processors and attached to computers using a convenient USB connection. FPGA miners used much less power than CPU's or GPU's and made concentrated mining farms possible for the first time.
Application-specific integrated circuit (ASIC) miners have taken over completely. These ASIC machines mine at unprecedented speeds while consuming much less power than FPGA or GPU mining rigs. Several reputable companies have established themselves with excellent products.
For a comprehensive comparison of bitcoin mining hardware, visit bitcoinx.com.