- v0: global coverage, low detail. This is "the last resort", but unfortunately it is the only layer that covers the entire world.
- cs: it is v0 with some contributions someone did in the past. Get this instead of v0
- v1: only parts of some countries available, higher detail than v0
- clc06: only Europe. Very high detail. This is the preferred choice for an European country.
- osm: global coverage, high resolution but very low detail. This means: if you are interested in an area out of Europe and forests are defined in OpenStreetMaps for your area, this is going to be the best data source. If forests are not defined, you will get a dull, neverending DEFAULT landclass. osm provides the best coastline and I encourage you to use this coastline even in Europe.
As far as I know, currenly you cannot contribute back to any layer. We don't have the resources to manage this. The v0, v1 and clc06 layers are read only, they come from external institutions and we are not going to change them. cs means "custom" and this is the layer that would contain user's contributions, if we had the resources (and we don't)
Your best bet is collaborating with OpenStreetMaps in your area, defining roads and buildings. Green areas are usually defined by the local authorities, but you can try to define them if you are in the mood:
http://www.openstreetmap.org/ Any change you do in that webpage will eventually appear in the osm layers. It is not automatic, you have to wait weeks or even months. The good news is you can download OSM data as SHP layers from many other sites:
http://wiki.openstreetmap.org/wiki/Shapefiles For example:
http://download.geofabrik.de/