No, it’s the International Space Station (ISS).
The ISS is one of humanity’s greatest scientific and technological achievements, and a powerful symbol of international co-operation. It is also the third brightest object in the sky (after the sun and moon). If you have seen it without knowing what it is, you probably assumed it was an aircraft (although a very high and fast-moving one; it orbits at an altitude of over 200 miles at a speed of about 17,000 mph).
The light you can see from Earth is sunlight reflected from the ISS’s solar panels. For that reason, the ISS is visible with the naked eye just after dusk or just before dawn, when the sky is dark but the ISS is still in sunlight (it also helps if there’s no cloud). An overhead passage lasts about two to five minutes.
To find out when the ISS will be overhead, there are two good sources of information. First, go to NASA’s Spot The Station site and register for free e-mail alerts. Second, n2yo’s Real Time Satellite Tracking site provides predictions for the next five days, including maps.
It is fashionable to sneer at this sort of thing, so don’t go outside looking for the ISS if keeping your badge of ‘cool’ is important to you.