1

Is it possible to get a single area from the server ( rectangular shaped ) from the server without downloading the whole planet.osm ?

2

okay, i found something http://api.openstreetmap.org/api/0.5/map?bbox=11.54,48.14,11.543,48.145
is this what i am looking for ?

3

http://wiki.openstreetmap.org/index.php/Osmxapi

4

You can also get smaller areas of the planet, look through the mirror list

e.g. Europe is here:
http://download.geofabrik.de/osm/