Each map is a complete street level map for your iPhone. The entire map is stored on your iPhone, so it works where you have no access to the Internet - it works where Google Maps is inaccessible.
This prevents expensive data roaming charges by loading all the map data to your iPhone before you leave home. You can even switch your iPhone to 'Flight mode' and still be able to find your way around.
Lets you find all streets, and many useful locations such as ATM's, parkings, museums, and many others.
Remembers where you've been so you can find that parking spot where you left your rental car.
To make sure your phone doesn't use any expensive data on your trip, go to Settings, then General, tap on Network, and slide the Data Roaming to "off".
Note: GPS functions work on the iPhone only