The names of the tables should be self-explaining:
- planet_osm_point is a point layer file
- planet_osm_line is a line layer file
- planet_osm_roads similar to line, contaning simplified geometries of roads and railways for lower zoom levels
- planet_osm_polygon contains (multi-)polygons
These tables reflect the OSM data model in the way they can be used GIS software, and for rendering.
You should also have
_rels. These represent the OSM data in the same way they are stored on the OpenStreetMap servers for editing.
Building information should be stored in the polygon table. If you want point information, you have to build centroids on the building polygons.
If you need address information, you also have to look into the point table, because the OSM data model allows address information to be put on POIs too, if there is no building polygon, ore one building has several housenumbers.
Furthermore, there is the possibility to have address information in the form of lines used for address interpolation.