Normal traffic lights.

The traffic lights come in two forms that are most commonly used, three phase or four phase. They come first in the following table. Also in most countries the lights are always vertically aligned, and in that case the red light is always on top; this is done to help the colourblind. (An interesting anecdote. Back in the 70's Hungary issued a postage stamp with a traffic light on it. It was upside-down.)
The standard three phase traffic light.
Green means go.
Yellow means stop, unless that would be dangerous.
Red means stop.
Supported by the Vienna convention.
The four phase traffic light.
The additional phase (red + yellow) means prepare to continue.
Supported by the Vienna convention.
This version was in use in Sweden until 1999. Yellow is replaced by green + yellow with the only meaning that red would come up.
A traffic light as found in the former DDR.
There is an additional phase between green and yellow, indicating that the light will go to yellow. According to the law of 1970 as modified in 1991, these lights remain valid but the green+yellow phase should be interpreted as green.
A traffic light as I saw them in Saint Petersburg in Russia.
Yellow is replaced by green plus yellow, and there is an additional blinking green phase; the meanings are probably the same as in the previous example.
A traffic light as could be found in Belgium.
The yellow light is removed and is replaced by red + green, in all other aspects it is just a three phase signal. This kind of light is no longer in use since 1973 according to the rules of 1968.
Apparently this kind of light is (or was) also used in Brasil.
Another light from Belgium, the green phase is replaced by green with blinking yellow, it means that there is still danger. I have never actually seen such a light, but it is one defined by the law.
This light can be found in France.
Green is replaced by blinking yellow, meaning that you may continue but have to give way to crossing pedestrians and traffic.
This one can be found in the UK and Ireland on pedestrian crossings.
Red + yellow is replaced by blinking yellow, meaning that you have to give way to pedestrians that are already on the crossing.
A continuously blinking yellow means that the signal is not functioning normally and that you should proceed with care.
Supported by the Vienna convention.
In Northern America a flashing red light can be seen. It signals that you have to come to a full stop and may proceed only if there is no crossing traffic that normally would have priority.
This or similar sequences are used in many countries on places where the cross traffic is not very frequent, and can for instance be used on some pedestrian crossings where the pedestrians govern the ligths with a push button. I have seen the lights mounted with a dummy third face, as shown here. They can also be mounted with only two faces. Also the flashing yellow before yellow need not be present, and there can be yellow + red and/or flashing yellow after the red.