if we are talking just about heating water, it is all about the equations of phase transitions. i.e a liquid will turn from a liquid to a gas (boil) along certain curves of pressure and temperature, and back again.
generally, the more pressure, the harder it will be to boil something, and higher the temperature, the easier it is to boil. the relationship is not linear, but there are equations that govern it. i cant really explain it more than that, because i didn't pay that much attention in therodynamics at uni, it was one of the subjects that didnt interest me much.
another good example of the phase transition/pressure thing is propane gas.
at room temperature, it is a gas, but when you put it in a bottle, i.e compress or pressurise it, it changes state into a liquid.
this gets complicated by the phenomenon called latent heat. latent heat is the amount of energy required for a substance to change state.
maccatak11 wrote:Another is shut the **** up.
should have waited till i finished typing this post as well

if you cool water down very slowly, gradually lowering the temperature, you can actually keep it in its liquid form, but get it to a point where its temperature is lower than 0 degrees, because the water hasn't lost enough energy to change state into ice yet.
sorry for nerding that up a bit, but i find it interesting