Planer

A thickness planer feeds a board through a set of rollers past a rotating cutterhead mounted above the bed, removing a controlled amount of material to bring the board to a consistent thickness across its whole length. It's what turns boards from three different suppliers, three different rough thicknesses, into stock that all matches.

The one thing people misunderstand about it

A planer does not flatten a board. It mills a consistent thickness relative to whatever face is riding on the bed — if that reference face is cupped or twisted, the planer reproduces the same problem on the opposite face, just thinner. Flattening is a jointer's job (or a hand plane's, or a router sled's); the planer comes after that step, not instead of it.

Benchtop vs. stationary

Benchtop planers (roughly 12-13" width capacity, portable, single-phase) cover the vast majority of home shop needs and store away when not in use. Stationary planers (15"+ capacity, heavier castings, more horsepower) matter for regularly milling wide stock or production-volume work — overkill for most people starting out.

Straight knives vs. helical cutterhead

Straight knives are cheaper and leave a clean finish on cooperative, straight grain. A helical (spiral) cutterhead — many small indexable carbide inserts instead of one long blade — handles figured and reversing grain far better, runs quieter, and shrugs off an occasional embedded staple that would nick a straight knife. It costs meaningfully more, which is the entire tradeoff.

Snipe

Nearly every planer takes a little extra material off the first and last few inches of a board — "snipe" — unless the board is well supported on infeed and outfeed. Good technique (long infeed/outfeed support, letting the board settle before committing to the cut) matters more here than any single feature, though some machines include a snipe-lock that helps.

Safety

Keep hands well clear of the infeed opening — the rollers pull material through with real force, and a planer will not distinguish between a board and a finger that gets too close. Never reach into the machine while it's running to clear a jam; shut it off and unplug it first.