There are many GPS receiver models, most made by Garmin, Tom Tom and Magellan. They tend to have the same basic tools but can vary in storage space, auxiliary tools, upgrades and more. Prices tend to range from around $100 to $200. Check out these various links for more information and to compare models.
In order to work a GPS, you simply enter into the GPS the longitude and latitude coordinates of the geocache you want to search for as a waypoint and then, no matter your location, begin searching. Follow the readout to the area your GPS leads you and search for the cache, using the clues, if required.
There are various methods of inputting coordinates into your GPS system:
If your device does not connect directly to the computer via an interface cable, you will need to enter coordinates into the device by hand. This process will be different for each device. Check your device's manual for instructions.
Send to GPS
If you have a DeLorme, Magellan or Garmin that connects directly to your computer through an interface cable, you can use the "Send to GPS" functionality to send a cache listing directly to your GPS device. The first time you use this functionality, you will be instructed to download the appropriate plugin for your GPS device.
Download CSV file
If you have any model of GPS device with an interface cable, you can download the cache listing as a file.