Noctilucent or Night shining clouds, often called NLCs, are the highest cloud layer. Formed from ice crystals, they appear from around mid-May to mid-August. They look a little like cirrus clouds except that they appear to shine before dawn and after sunset. The reason for this is that they’re catching the light of the Sun below the horizon and reflecting it down to the ground.
It is difficult to predict exactly when and where these clouds will appear (except that chances are highest during the season mentioned and improved the further north you are), but it does help to have a group of observers ready to send out alerts when activity brews – twitter is one help during times like this.
Radio 4 has started up a series of amateur science experiments during the summer of 2010 that can be performed by listeners. One of which is to make observations of NLCs. Typical observations would include their spread across the sky, their morphology and their brightness, all of which can vary tremendously. Some of my photos of NLC activity last year (which began in mid-June rather than May) can be seen in posts like this one and this one. This page has information on types and how to observe NLCs properly.