Something that's intensive means it's very involved or rigorous.
Extensive means something is far reaching in its affecting other things.
The first two phrases that come to mind in using these is "labor intensive" and "extensive warranty"
If something like a car is very labor intensive, it means there's a lot of work that goes into making that one car and it might take something like 1000 man hours to make. On the other hand, a car that's not labor intensive might be able to have been put together in 24 hours by some machine.
Car dealers often offer "extensive warranties." Which means your car is covered for a wide range of problems, from replacing tires, free alignments, roadside assistance and so on and so forth. Here "extensive" is describing how the warranty covers many different problems.