Hey FK270673: The distinction is set out on the wiki page here: osm.wiki/index.php/De:Map_Features#Fahrradwege
cycleway=lane means Fahrradstreifen (separated by paint from the car lane)
cycleway=track means Fahrradweg (separated by the kerb or parked cars from the car lane).
Despite Andy Allan's comment, I suspect a lot of mappers in Germany do distinguish a third situation - a bikeway five or six metres separate from the roadway - and tag it as highway=cycleway. That makes sense. These bikeways behind thickets of trees are much more pleasant to use, and also safer for children.