Solid or engineered?
One of the first things to think about when you are selecting your hardwood floor is wood species. Species determines grain, hardness, and color. There are many variations of each.
For example grain can be a streamline select grade or a rustic grade with lots of color variation or knots.
Hardness is compared to the standard oak: harder or softer than oak.
The natural color of the wood is common because there are so many beautiful natural colors, but woods are often stained as well.
Character Grades: An important difference between companies is the character grade they supply. It isnt accurately depicted in a small sample, the amount of color variation and knots in a floor. The heart wood, the very center of a tree, is the oldest, most stable, most color-consistent part of the tree. Those pieces cost more and will be reflected in retail price. It is not "bad" to have color variation or more knots, it's just a personal preference. A few companies, like Mirage, offer options. Mirage names them Traditional, Exclusive and Select & Better. Contractors know it as #2 common, #1 common and select. It can vastly change the look of the floor.
Red and White Oak are the most commonly used hardwood in the US. The two are distinctly different and yet are often confused
White Oak Red Oak
Maple, Beech, Birch are the lighter colored domestic woods. Maple is pretty consistant in color ranging from white to yellow, beach has a slight pink tone, and birch is white and browns and can have much color variation in the character grades.
Walnut and American Cherry are favorites because of their flowing grain and beautiful brown/grey and golden colors (respectively). They are, however, some of the softest in density of the hardwoods and are prone to denting.