When I take a picture through a window, I often see a reflection on the glass. I have noticed that this occurs mostly when it is dark or overcast outside or it is especially bright on my side of the glass. How can I stop this?
This question has been asked in many guises, ultimately you want to prevent light from inside the room reflecting off the glass and going down the lens.
This can be done by moving the lights, so the direct reflection misses the camera, or more preferably (especially if it's a well lit room and you have light bouncing off everything in many directions) blocking the light.
You can block the light with your body, or get the camera so close to the glass that there is no way for the light to get in.
The easiest way would be to use a polarizer filter, which allows light coming from a single direction but blocks most of the light coming from the other directions.
Depending on your camera model that could be harder to do. Usually bigger cameras have adapters for filters built in the lenses, but some smaller cameras also offer this option, just not all of them.
So you could use to it to capture the light coming from the direction of the scene itself, but block most of the light coming from the others sources behind the glass (the reflections you are seeing).
As Matt Grum noted, if you can remove or block the light from inside you will have a lot less reflections. But even if you don't have lights turned on behind the window, the light that comes from outside may reflect on you and your background and appear on the window (therefore on your shot too).
Again, even if you had no lights behind the glass, the polarizer would allow you to capture mostly only the light coming directly from outside.
Several good answers have been given, but for the larger issue of understanding light, how it reflects, and how it interacts with photos, I highly recommend the book Light: Science and Magic.
There are sort of two answers here.
The first, if you can control the orientation of the lights in relation to you and the glass, you may be able to avoid reflections. To go along with that, you could possibly use a go-between (or gobo), a piece of black paper or cloth that can possibly block the stray light.
Second, you could use a polarizer filter (circular polarizer for a dslr) to filter our the stray reflections. This has the upside of being very portable and puts you in control, it has the downside of loosing a stop or two of light (making for slow shutter speeds), which can be bad for indoor or low-light photography.
Check out the book "The Science of Lighting and Magic" if you really want to get into the details of angles and reflections.