Satellites. When they roll over and catch the suns light they flare a little. Brighten up and then fade as they move pass your horizon. They are traveling around 18,000 m an hour.
I believe there is a site that tells you when they are going overhead and the direction they are traveling in. You can follow the space station this way also. It shows up fast and within about a minute or so it's already out of view. Depending on where it's position it can be pretty bright some nights.