offline - How to use GPS/Maps without an Internet connection?

I need a way (possibly a third-party application) that allows me to use maps and GPS without an Internet connection.

I don't use it by car, just for walking in cities. It would be nice if I could just copy all data relative to a particular city before going there for tourism.

The Google Maps app does not store maps reliably, so it is risky to rely on it without an Internet connection.

Answer

I am now using OsmAnd.

It stores OpenStreetMap maps on the phone's storage.

OsmAnd uses a vectorial format, which means it can store a lot of information without taking too much space, and is visible in all zoom factors.

You can download the maps you want, for instance Italy, Bolivia, or any other country/region.

Free, open source.