Solid wood is just that – one piece that is milled from lumber. It comes in a variety of widths, from 2-1/4" to 5" wide. It also comes in different thicknesses: 3/4" is standard, but you can also find "thin profile" solid that is 5/16" thick.

Moisture and extreme temperature changes can cause solid wood to shrink and expand, potentially causing gaps between boards during colder or dryer seasons.
5/16" thin profile solid wood can be installed directly over concrete; 3/4" can't.
All solid hardwood can be sanded and refinished if needed (required every 15-20 years).
All urethane coated floors can be recoated as needed to renew the surface of the floor. This is more economical than sanding and refinishing.

Engineered Hardwood

Engineered wood is real wood, but instead of one solid piece it consists of three to 10 thin layers of wood, called plies, that are assembled and glued in a cross-ply construction. The top layer of wood can range from 1/16" to 1/6" thick. Overall board thickness ranges from 1/4" to 3/4". Due to its multi-ply structure, engineered wood is much more stable than solid wood and is less susceptible to shrinking and expanding with changes in temperatures and humidity.

Engineered wood can be installed directly over concrete and can also be installed below ground level.
Engineered wood with a top layer thinner than 1/10" cannot be refinished; however, it can be recoated to renew the surface of the floor.

Site-Finished & Prefinished 
 
Prefinished
A pre-finish, or factory finish, creates a no muss, no fuss installation. There's no dust from sanding, no fumes from chemicals, no waiting for the finish to dry. These floors are warranted to last longer than site-finished floors–typically 15 to 25 years, versus three to five years. This is because factory applied urethane finishes are much tougher and longer lasting than site-applied finishes.

Site-Finished
Advocates of site-finishing like the fact that the floor can be sanded after it's installed to even out any imperfections. Site finishing also gives you the widest selection of stain colors to choose from. However, on-site sanding and finishing can produce heavy fumes, messy dust and significant waiting time from when the floor is laid until it can be enjoyed. In addition, labor costs for on-site finishing may drive up your installation costs.

Prefinished floors are warranted for a longer time period than site finished floors.