Environmental stickers (Feinstaubplakette) are needed to get into German cities that have introduced environmental zones (Umweltzonen). The sticker, which shows that the car's particle emissions are below a certain level, must be mounted on the inside of the windscreen. If you are not going to enter an environmental zone but only drive through Germany on Autobahn, no environmental sticker is needed.
Vehicles that are not marked with an environmental sticker are not allowed to enter an environmental zone. This also applies to vehicles that are registered in Sweden or another country. Should you still drive into an environmental zone, you may be fined and the vehicle is not allowed to be driven from the place where you have been stopped. The vehicle must be towed from the environmental zone for further travel.
Environmental zones are signposted on traffic signs with an additional sign indicating which color stickers that are allowed to enter the area. The environmental sticker is available in three colors (red, yellow and green) and which sticker to use depends on how the car meets European emission standards. Environmental stickers can be ordered from DEKRA or be bought on site in German car repair shops or inspection stations.
Read more about environmental stickers here.