As a satellite traverses the sky, a few of them (including most if not all of the Iridium Satellites) exhibit a sudden flaring or glinting of the sun off of the shiny aluminum solar panels. This can be dramatic a times with some of the flares strong enough to cast a shadow at night.
This is a flare from Iridium 6 that occurred tonight just a few hours ago. It is a 53 second exposure, ISO 1000, f/4.5 at 20mm, Canon 60Da camera.

Added second satellite taken next night. Almost in same area of sky but this is a cropped image. Taken with same camera but 30 second exposure at f/5.6, ISO 800. Cloud cover seemed to enhance the satellite but it should have appeared much dimmer. This goes to show that even the best predictions can be off somewhat when it comes to how bright an object should appear.