I will divide strip floor from plank floor in this discussion because they have different fastening and sub-floor requirements. Tongue and groove strip floor in 2 1/4" or 3 1/4" wide and 3/4" in depth, is probably the most popular hardwood flooring available to North Americans today. The most common species used is red oak, in a flat sawn select grade. It is moderatly durable and it's contrasting grain pattern gives a textured look to the floor, which helps disguise scratches and wear and tear. Lengths vary from 1 foot to 6 feet with an average size of 3 feet, allowing a ranomized joint arrangement. Be carefull if you choose a lower grade than select. I have found that common #1 and #2 grades these days include many internal cracks and loose knots that will cause these floors to self detruct in less than a decade. A better choice is a mill-run grade (that contains all the grades) from a really good hardwood mill. You can save some money and ferret the bad pieces out and install them in the back of a closet.
A strip floor requires at least a 3/4" plywood subfloor for it's 2" nails to be fully engaged. Never use OSB, chipboard, or any type of particle board, as the nails will loosen and floors will squeak. Try to run the hardwood floor at right angles to the floor joists, this will give a decided firmness to your new floor. In older houses, a well nailed softwood plank sub-floor at least 3/4" thick is also very good. But be sure to run the new strip floor opposite or diagonal to the run of the sub-floor. Nail at least every 8" with mechanically driven Power Nails ®, they are simply the best. The first few and the last few rows need to be hand nailed with 2" flooring , or finishing nails. Pre-drill or use a nail spinner (made by Vermont American) to avoid cracking the stips. Never use air driven 15-18 ga. staples or nails as these are wholly inadequate for these heavy duty floors.
Strip floors come in 3/8" and 1/2" thicknesses also. I do not recommend these as in a tongue and groove board they only can be sanded 2 or 3 times before they wear out. The exception to these thinner boards is the laminated plank or "engineered flooring" which have many unique features. It's the only wooden floor that can be laid below grade, and can be glued directly to concrete. Don't try to sand and refinish these floors, you will quickly sand through the thin top ply, I know I've done it. They can be recoated or chemicaly stripped and refiished. Square edged flooring is still used in some parts of North America, it has to be face nailed and all these nails set and filled before every resanding. It's not my favorite job but it's a very durable type of floor in any thickness.
Plank flooring is a different animal altogether. When 4" or wider boards are installed you must have at least a 3/4" plywood subfloor if you are crossing the joists at right angles, but 1" thick if you run the plank parallel to them. Like strip floor it needs proper flooring nails ( no staples please) every 8" and because the nails are farther apart in this type of floor, screws are needed also. Use 2" screws counterbored into the ends of all the boards, two at least, and three if the planks are 8" or wider. This will prevent warping and some of the gapping that occurs with these floors. The counterbored holes are filled with wood plugs of the same species or a contasting one like walnut. Remember the wider the boards will react to large humidity changes by forming large gaps in time, so this may not be a good choice for a cottage floor that is closed for part of the year.