The tracks visible in these clouds off the coasts of France and Spain form when small, airborne sulfate particles emitted by ships and airplanes act as cloud condensation nuclei, or "seeds." The accumulation of these seeds forms the thin streaks of clouds seen here.
Clouds formed by these particles tend to be more reflective and carry more water than usual, but they don't release much precipitation. In the aggregate, these clouds could have a big impact on the global climate.
This image was taken on January 27, 2003, by the MODIS instrument on NASA's Aqua satellite.