Broadly speaking, lawnmowers come in two styles---the reel, typified by your venerable unpowered push mower, and the deck, or rotary, which describes most of today's powered, walk-behind machines---so deciding between blade designs is simple. Some brands of deck mowers accept several types with slight differences in performance and function. Still, mower blades made by different companies do not fit all types of machines even if the deck measurements are the same. Choices for older machines could be limited.
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Still appropriate for small yards, the wheels of push mowers drive a blade cage revolving against a cutting bar with a fast, scissoring action as owner-operators provide the motive force. Sharp blades make a noticeable difference in ease of operation. The only blade option is the one that fits, so replacing the mower could be as reasonable as replacing the blades.
Deck mowers cut with many variations of the same basic blade---a flat horizontally mounted steel bar that revolves like a fan blade and cuts with both leading edges. Blade shape and mounting hub could be uniquely shaped to fit one company's model of mower. Universal mounting blades do not fit all machines. Your best source for replacement blades or alternative blade designs is the company that made the mower.
As the rotary mower passes across your lawn, the deck presses down the blades of grass. Air pressure from the cutting blades may keep the grass down, resulting in a ragged cut. In heavy grass, tufts and rows of partly cut grass often lift up behind the mower. With lifting blades, a section of the blade's back edge turns up toward the deck. The draft generated by this blade shape pulls grass up, toward the cutting deck, and improves the quality of the cut. Blades are available in low-lift and high-lift designs for some machines.
Mowing a healthy stand of grass creates a large amount of clippings. Left in clumps on the yard, the clippings can suffocate and kill small patches of lawn. To reduce this problem, mulching blades use high-lift designs with added features like notched or rake-toothed lifting edges. Because of this extra blade action, clippings revolve beneath the deck for a longer time. The mower chops them into a fine mulch that disappears between grass stems and fertilizes the lawn.
While high-lift and mulching blades are an improvement over the simplest blade designs, do not expect perfect results. Mulching heavy or damp grass often clogs a mower deck with clippings, bogging down or stalling the machine. Cutting several inches of tall grass will not yield a clean-cut, finely mulched lawn in one pass. Adjusting mower height downward by one or two inches per pass gives better results. Dull blades exaggerate all problems.