When selecting hardwood flooring, you must decide between SOLID or ENGINEERED constructions.

Solid, like it sounds, is milled from a single 3/4" thick piece of hardwood. Because of its thickness, a solid hardwood floor can be sanded and refinished over several generations of use. One of the characteristics of solid wood flooring is that it expands and contracts with changes in your home's relative humidity. Normally, installers compensate for this movement by leaving an expansion gap between the floor and the wall. Base molding or quarter round is traditionally used to hide the extra space.
Engineered is actually produced with three to five layers of hardwood. Each layer is stacked in a cross-grain configuration and bonded together under heat and pressure. As a result, engineered wood flooring is less likely to be affected by changes in humidity and can be installed at all levels of the home.
Before deciding on Solid or Engineered hardwood flooring, consider these factors:
Location, Location, Location
The location of your hardwood flooring basically falls into three
categories:
What type of subfloor do you have?
If you plan to install over concrete, you must use an engineered
product to ensure structural integrity. Solid wood flooring or Engineered
flooring may be used over plywood, wood, or OSB subfloors.
Will there be moisture in the room?
If so, you'll want to select an Engineered hardwood. The moisture
resistance of an engineered hardwood makes it suitable for rooms
where moisture is a possibility, such as bathrooms.