The 'What' of a crawl space is: primarily it's a band-aid fix that creates problems for the life of the building, but the specific definition is it is the space between the ground and the subfloor of your home.

Why a crawl space- to separate a structure from the ground, which provides space, or should provide the space, to install and service utilities such as water lines, heating systems, etc.

When: For the most part, unless a home is built on a slab or has a basement it will have a crawl space under it.

Note: though you may have to crawl in an attic, an attic is the attic. The crawl space is what's under the house. I keep having folks call up and start telling me about problems in their "crawl space", when part way into the conversation I realize they mean their attic. If by chance there are some trades people out there, who are starting to use 'crawl space' for attic- Stop it! If the attic is the crawl space, then what do we call the crawl space? So, let's keep it straight, the attic is up, the crawl is down.

More details on what a crawl space is. When we construct a building there has to be something that separates the structure from the ground. Wood has been our most plentiful and economical building material. But wood has some drawbacks, in the presence of moisture and air, it deteriorates, and quite quickly. Also, when wood is exposed to high of moisture, even if that moisture is just in the air, it becomes susceptible to mold, dry rot, wood infesting beetles, carpenter ants, termites, moisture ants, etc. So we need to separate any wood from making contact with the ground. Of course there is pressure treated wood, but that's not cost effective and has its own problems also.

In the old days, to create this separation a home was build on post and pillar, which meant a home sat on stilts to hold it up off the ground. In this type of construction there wasn't even a foundation. Sometimes there would be pillars of stone at the corners, then posts under the interior of the home and sometimes the home was built entirely on post. It was a very cost effective construction style, but only in connection with the initial cost of constructing. As far as maintenance and longevity it was a terrible building style. Everyday people in Europe are buying and living in homes that are 100-200 and more years old, why? Part of the reason is because they used stone and stone like products at the point of contact with the ground.

If you want anything quick and cheap up front, you pay for it in the long run.