There seems to be some confusion in the general population between Sat Nav and GPS.
GPS or Global Positioning System refers to the array of Satellites in various orbits about the Earth. You use a GPS receiver to receive signals from these satellites. Many of these GPS receivers are about the size and shape of a largish mobile phone (By today's standards) and they display your position anywhere on Earth. In the old days (about 10years ago) this display showed your location in degrees, minutes and seconds of longtitude and latitude. You really needed a paper map to see where you actually were.
Many of these GPS receivers now have base maps of the whole planet on them showing coastlines and major roads and cities. Your actual position shows up as a cursor on this map so you can actually see where you are. You can buy more detailed maps for many parts of the world, (most of Europe, apart from Greece and some of the East European states further North, although they are working on these regions as we speak) which you install on your device as software.
So anyone with a GPS receiver with a base map of the world will be able to see where they are relative to the coastline,and if they're lucky relative to a major road or town. They are useful in that you can plot your track on these and if you get lost, simply backtrack.
Sat Nav or Satellite Navigation to use its full name, uses the same GPS satellites and its receiver is a GPS receiver, but it has the added capability of guiding you along specified routes or between different points. It usually uses a voice to guide you as well as markers on the screen. It too needs maps in the form of software, but these are usually dedicated mainly to surfaced roads, and you have to buy these maps for different regions, eg England, Scotland, Wales and Ireland, France and Spain etc. They are no good if you stray from the roads that are shown. They will usually ask you to return to a road so that they can show you your position.
Several companies are working on "mapping" Greece for these gadgets, and they have started with the big cities naturally, where most of the people live, so you can get some coverage for parts of Greece. Don't hold your breath if you're relying on them to show you around Corfu though. When they do arrive in Corfu, there is no guarantee that they will show the road network as it really is, (None of the mapping companies have bothered with paper maps yet) and only the major roads may be shown. Some minor roads and probably all unsurfaced tracks will be "invisible" on the display, so they will not be all that useful.
I think there may be a future for the Corfu map for a few years yet.