Solid hardwood is true to its name in that it is constructed from solid wood, from the same species, whether it be oak, hickory, cherry, or some other species. Species can include both domestic as well as exotic woods, and you can find them prefinished, or you can choose to have them unfinished.
A standard solid hardwood plank is usually ¾ of an inch thick. This gives plenty of room for refinishing. In fact, most hardwood floors can be refinished about seven or eight times before needing to be replaced completely. This lends to their extra long lifespan.
It is important to know that hardwood both contracts and expands based on the humidity and temperature. Because of this, it is not advised to be used in basements.
Engineered flooring is manufactured by bonding several layers together to form one single plank. It is very stable because the plies are stacked in alternating directions. The layers on the inside are comprised of plywood, hardwood and fiberboard, while the top layer is your desired species of wood.
Neither humidity nor moisture affects engineered hardwood the way it does solid hardwood and it is cheaper than solid wood as well. You can install engineered wood over many different types of subflooring. However, it cannot be resanded as often as solid hardwood can. In fact, if the veneer is less than 2mm thick, or if it is hand scraped, it cannot be refinished at all.