Your GPS receiver has a GPS chip that receives signals from satellites (at least 4 required, read How GPS Works) and uses those signals to get a precise fix on your current location.
But it needs to do more than this to be useful as a navigation system.
Inside your GPS receiver is a map that it uses to plot your precise location. Now, it has a reference point — your starting location.
When you input your destination, it uses the map and sophisticated navigation software to plot a route based on your navigation preferences. It calculates a route from your starting position to your destination based on its navigation algorithm.
Map & Navigation Software
This means that the quality and freshness of the map, plus the local navigation knowledge used by the software, will determine how useful your GPS receiver will be.
For example, if the map used in your GPS receiver is not up-to-date and you live or drive in a relatively new development, your GPS receiver may not be able to help you too much there.
For example, it may lead you to the street but not the exact house number (assuming the houses were not built yet when the map was last updated); or, it may lead you to a cross-street (intersection); or, it may not be aware that a certain street is now one-way or closed to circulation.
Also, a GPS receiver that primarily relies on maps and route data based on published information will not be as accurate as one that supplements this data with local knowledge about the roads: a route based on local traffic knowledge will always be more accurate than one determined by an algorithm that is not aware of local conditions. For example, it may not know that a certain road is always congested at a certain time of day; that the main entrance to an amusement park is on a different road than the postal address of the amusement park; and that a simple detour could shave minutes from your drive.
Some GPS receivers have more useful features than others.
For example, one very practically useful feature is the ability to do Advanced Planning, allowing you to get a feel before getting on the road for the route, distance and time taken for a particular trip. This allows you to decide whether a trip may be too long — and require a break and a rest stop (perhaps even a night stay at a hotel) along the way.
Being able to create an Itinerary is also a very handy feature to have. This is especially true if you are planning a multi-destination trip. You would want to be able to save each destination, and create an itinerary that leads you from one destination to the other during your trip.
Useful information that you would want to see at a glance on your screen (without having to interact with your receiver while driving, which can be dangerous and is therefore not rercommended) include current street (driving in an unfamiliar area, knowing you are on the right street gives much confidence), next street to turn into, next direction (e.g. is the next direction a turn and how far away), time to arrive and time of arrival (sure, we can do the maths but we’d rather reserve any mental energies to concentrate on the driving).
Learn more about GPS receivers and compare models: