What does hot and cold mean on an AC outlet?
In the USA

One thing that the other answers did not cover, the line coming in to your house is actually 3 lines from a transformer with a center tap. That center tap is the "cold" and the two ends are both "hot", to get 120 you go from the center to one of the ends and to get 240 you go from end to end



In your breaker box it alternates which end is available each line, that is why 240 breakers are two wide, it is connecting the two ends together instead of the middle and the end.





In Europe

Similar principles apply with the following major differences


Most households are supplied with a single phase of 230 V AC
A three phase, four wire (+ protective ground), star topology network is commonly used.
The wiring colours are different (below for Netherlands, probably other countries too).


green/yellow striped is protective earth.
brown is live supply
black is switched live
blue is neutral return.