Active Heat
Active heat is heat that is generated by various combustion processes.  Some examples being car engines, the heating of buildings through the burning of gas or using electricity and factory heat emissions.  Even electric lighting gives of a considerable amount of heat.

Passive Heat
Passive heat is the heat generated by sunlight that is absorbed by buildings, roads, concrete footpaths etc..  If you have ever walked barefoot on a footpath on a hot Summers day you will understand the capacity of materials such as concrete to absorb heat.

These man made materials effectively act as heat banks. When temperatures drop at night or when the weather turns cooler they will slowly release the heat stored in them.  The more built up the urban environment the bigger the influence that these heat storage sources have on the prevailing weather pattern. My estimate is that it can increase the average temperature by as much as two degrees when compared to the surrounding countryside.