Engineered wood boards come pre-finished and are laid as a floating floor. They don't need to be glued or nailed to a base. They usually come with tongue-and-grove edges that are fitted together like puzzle pieces as you lay them. You glue along the edge of the board that's already down, then ease the other board into place along side it. Some of today's engineered boards also have edges that "click" together, much like laminate flooring, and they don't require glue.
There is more than one kind of engineered wood. Some types have a
high-density base with a very thin wood veneer, and they're cheaper than other
types, though still more expensive than laminate. You get what you pay
for, and these cheaper versions are less durable than other engineered wood.
Solid Wood Boards
Solid wood boards come in random lengths with tongue-and-groove edges for nailing and gluing, and they fit together much as engineered wood boards do. (In the old days, all solid wood boards came with straight edges, which had to be butted up closely to each other and glued to the sub-floor. You may still encounter these kind of boards if you work with reclaimed or salvaged flooring. Labor and installation costs may be higher since it requires more effort to install.)
Solid wood boards often come unfinished, so you may use a varnish, oil, or wax that you like. However, you can also buy pre-finished wood, just as with engineered boards. It's just a matter of your preferences and cost (unfinished wood is generally less expensive, but if you don't fancy the idea of regularly reapplying a varnish or waxing your floors, pre-finished may be a better fit for you).
Sometimes solid wood boards are laminated together into 2- or 3-strip-wide planks that are treated as solid single-strip boards. These planks are easy to mistake for engineered boards, but they're heavier than engineered wood, and that makes them hard to nail or glue--however, you can lay them on the existing surface to make a floating wood floor.
No matter which solid wood board you choose, be careful with the installation
because solid wood boards are more prone to expansion and contraction than
engineered wood. You want to leave a little room at the edges for the wood
to breath and expand. (Because of its plywood base, engineered wood is less
susceptible to this.)