robbo1869,
A GPS cannot plan routes and things like that, as you ask. Only a navigation system can do that. A GPS is simply a little device or chip that will connect to the GPS satellites and give you location, speed/direction if applicable, and altitude. Normally you'll feed this info into some sort of computer program which could then do interesting things like showing you a map and so on. The computer program combined with the GPS is often called a navigation system.

The N810 has a built-in GPS and comes with a navigation application and maps (to get routing with the navigation application you have to pay extra).

It's possible to use a different, external GPS with the N810 (any BT-GPS which supports the NMEA protocol), as well as additional software - e.g. Maemo Mapper, which is not really a full-blown navigation system, it's a great mapping system with some navigation capabilities. There's also an up-and-coming alternative piece of software for navigation, search the forums (I think it's called roadmap or something - I didn't look deeply, as it's for north america only because of the map sources).