How the heat works, in a nutshell.
The heater core is filled with coolant, and as the coolant temperature rises, the heat from the airbox does as well. There is a valve that opens and closes the hose that allows coolant into the heater core (controlled by the hot/cold slider). With the slider on hot, the valve is open, cold means the valve is closed.
If the coolant is not full, it may not flow through the heater core (effectively an internal radiator to warm the air) properly to do its job. Also, the thermostat may not be functioning properly and the coolant may not be reaching operating temperature (mentioned above). If the coolant never reaches the proper temp (180*?F), then the air flowing through the heater core will never get very hot. And the third option as Paden mentioned, is that the cable may be disconnected or the valve may not be opening fully.
And yes, your title sucks balls. Try a little harder to create a more useful title so we dont feel like we are helping a retarded person who doesnt understand the purpose of a title (the definition in this case is a DESCRIPTIVE HEADING).