what's the difference between different Garmin map types?

I bought a Garmin nuvi 275t GPS which includes maps of USA and Europe. I’m looking at the Garmin site and wondering what the difference is between the Navigator maps (e.g. North America…) and the city maps (e.g. CityXplorer, Bay Area) and the water maps and the topo maps.

Does anyone know what the overlap, if any, is? Or a web page / site that explains it?

It appears that I have the road maps for those two areas, the equivalent of the City Navigator North America and City Navigator Europe. Would I then have to buy the CityXplorer London if I’m going to London and want additional POI, transit maps, etc. or am I covered with City Navigator Europe?

While I’m on a roll – do the topo and street maps interact nicely with each other? Or do I have to switch between the street maps and the topo maps depending on what I’m doing?

TIA.