Hi Aaron, It's a 'clean air' windscreen sticker (vignette) which is now a legal requirement in some of its cities. The French have introduced it as a quick visual means to identify a vehicle’s emissions levels and in some cases, restrict access to some towns and cities in order to improve air quality. I believe that not all vehicles are eligible for the vignettes. The official website is
www.certificat-air.gouv.fr/ while the vignette is only €4 approx, The fines are fairly hefty...up to €140. I'm afraid other than that I don't know anything about the purchasing process outside of France or if the fines are only monetary or indeed if the vehicle can be seized.