The foundation is first piece of a home to be constructed and creates a base for rest of a home's components. There are three types of foundations that are commonly used in U.S.: slab, crawlspace, and basement.
Slab is a type of foundation consisting of a structural concrete slab poured directly on grade. No accessible space exists in slab construction. Slab foundations are popular in areas (i.e. Southern United States) where there is a relatively high water table. (Water table refers to depth in soil at which you find water).
A crawlspace is an accessible space with limited headroom, typically between soil and bottom of first floor of a home. Crawlspace construction is predominant in areas where there is heavy clay content in soil.
A basement is an accessible space between soil and bottom of first floor of a home. It usually has more headroom than a crawlspace. Basement foundation construction is predominant in cold climates where foundation needs to be situated below frost level.
All three foundation types are usually constructed out of concrete, but can also use concrete masonry units or insulated concrete forms.
Concrete Masonry Units (CMUs) are hollow, concrete blocks. To create foundation wall, mortar is used between blocks to hold them together, forming wall.
Insulated Concrete Forms (ICFs) are made of rigid foam insulation forms (a system of support assemblies, including mold, hardware, and necessary bracing to hold concrete) into which concrete is poured. Once concrete has gained its full strength, outside forms, inside forms, or both are left in place to insulate wall. ICFs are common in regions in which local building code requires foundation to be insulated. Another benefit is that homeowner or builder is able to finish basement immediately, without adding studs.
Choosing Type of Foundation:
Homeowners and builders make decisions about which type of foundation to use by gauging cost, needs/desires, and soil and weather conditions. If you have high water tables then it may not be possible to have a basement. If your land has shallow bedrock or boulders then it may be more costly to dig a basement. If you have a sloping lot it may be difficult to use a slab foundation. If you have a cold climate then you may need to dig down at least four or more feet to put home's footings below frost level. If you have to go at least four feet deep then it may be worth spending some extra money to dig a few feet deeper and have a full basement. Also, it is easier to install and maintain mechanical systems in basements (compared to a crawlspace). Your builder can help you determine what type of foundation is best suited for your area.