In the example shown above, heating components would be placed at one end of the tank, producing higher temperatures at that end of the tank than at the low end, where no additional heat is provided. Please not this is a completely fictional example since the photo is of my hermit crab tank and the temperatures given are simple an example of ideals for a corn snake (not what I provide for my crabs!).
If you have a species that also requires UVA/UVB lighting, that the UV producing light is placed in the same area as the heat source. Sometime the UV producing bulb will run the length of the terrarium, which is fine, as long as the bulb overlaps the basking spot/heating zone. This is to ensure that as your animal basks in the heat, it also gets to absorb UVB, which is what would happen naturally.
In the wild, most species experience a temperature drop at night, and this should also be considered. The temperature for most reptiles should be dropped at night to mimic natural conditions (check for your species for exact ranges). This usually means turning off some or all heaters, or switching to a lower wattage heat source at night.
