A docking station adds utility to your laptop computer by giving you the convenience of portability while you're traveling, but allowing you the ease of a desktop while you you're sitting at your desk. With a docking station, you aren't constantly plugging in and unplugging peripherals.
A docking station sits on your desk whether or not your laptop is there. You connect peripherals such as a mouse, an ergonomic keyboard, speakers or a printer.
When you sit down at your desk, you place your laptop on the docking station. Connectors on the docking station relay information from your laptop to and from your peripherals without plugging in a single extra cable.
There are port replicators on the market that attempt to duplicate the function of a docking station at a lower cost. These typically plug into a USB port on your laptop and may not function seamlessly, as they are not tailored to your particular computer and require drivers.
Laptop docking stations may be small devices that only connect to the bottom or back of your laptop. They may also be larger constructions designed to put your laptop screen at a different height or hide the entire laptop under a larger desktop monitor.
A laptop and a docking station are an expensive combination, together. For some people, the added utility is invaluable, but if you only need to plug in a mouse or change the height at which the laptop sits, it's probably worth finding another solution.