A workstation is a desktop computer and everything in the area that allows you to comfortably perform your work. Generally workstations include a computer, monitors, keyboard, mousepad and mouse and any peripherals (printer, speakers, etc). Workstations can be adapted to the sort of work a person is performing, sometimes a laptop is preferable to a desktop computer, in other situations a LAN connection is needed along with quality speakers, your workstation includes what you need to work.
Web Applications UK is a Dell and HP hardware reseller; both have some excellent options for workstations.
Dell has a range of Precision Workstations - each comes with a number of options. Customise your workstation to make sure you get what you need with variable Processors, Memory, and Operating Systems.
HP also has a number of Desktops available to get you started on planning what you need for your workstation with the HP Pavilion and HP Omni lines.