As stated here, noctilucent clouds are:

Clouds at extremely high altitude, about 85 km, that literally (as the name suggests) shine at night. They form in the cold, summer polar mesopause and are believed to be ice crystals. Because of their high altitude, in a very dry part of the atmosphere, noctilucent clouds are rather an enigma and are being studied by a number of people around the world.

More information on these clouds can be found at the NLC FAQ. Here’s a nice pic of some of these clouds.