Exterior open entrance to a metro station (Tribunal station in Madrid)
Interior of an underground metro station (Place-Saint-Henri station in Montreal)
A metro station is a train station for a metro. It is often underground or elevated. At crossings of metro lines they are multi-level.
At street level the logo of the metro company marks the entrance of the station (often a large "M"; for London Underground's circle with a horizontal bar through the center, see the picture there).
Often there are several entrances for one station, saving one from having to cross the street. In such a case, tunnels or overhead stations can often also be used just to cross the street.
In some cases metro stations can be connected to important buildings by a direct enclosed hallway (see underground city).
UrbanRail.Net (http://www.urbanrail.net) (formerly called metroPlanet) – descriptions of all metro systems in the world, each with a schematic map showing all stations.
Metro Bits (http://mic-ro.com/metro/metroart.html) Arts and architecture of metro stations around the world.