A Phase Diagram shows how temperature and pressure affects a substance in a closed container. All possible points on the phase diagram represents different combination of temperature and pressure. The diagram is usually divided into sections representing solid, liquid and gaseous states of the substance.
Phase diagrams are drawn distorted in order to make it easier to see what is actually going on.
The diagram above shows line representing all three phases and two special point C & T. The point represented by C is called the Critical Point (critical temperature and pressure). You need to carry out condensation below the critical temperature in order to obtain liquid. While, the point represented by T is called the Tripe Point; all three phases are in equilibrium at the temperature and pressure found at the tripe point.