Razor burn is mechanical and chemical irritation of the skin surface. Four things cause it: friction from too little lubrication, a dull blade, too much pressure, and alcohol applied afterwards. It appears within minutes to an hour of shaving, as diffuse redness and stinging over the whole area you worked on.
That timing separates it from razor bumps, which take one to three days and appear as individual raised follicles. Different mechanism, different timing, different fix. Treating one as the other is why people buy the wrong product.
What is physically happening
Shaving removes hair and, unavoidably, some of the outermost layer of dead skin cells that sits on top of the living skin. A well-lubricated single pass with a sharp blade takes very little of it.
Every mistake takes more. A dull blade needs pressure to cut, and pressure scrapes. Insufficient lather means the blade drags across skin rather than gliding over it. A third and fourth pass over the same patch removes what the first two left. When enough of that surface layer is gone, the skin underneath is exposed, loses water faster, and reacts. The redness and heat are that reaction.
Nothing you apply afterwards can put the layer back. The entire fix is upstream of the shave.
Cause one: not enough lubrication
The job of shaving cream is glide, not coverage. A thick white foam that looks impressive on the face can still be a poor lubricant, and canned gels that expand into foam are mostly propellant and air.
Build a lather with enough water in it that it looks slightly glossy rather than dry and stiff. A dry lather has no water to donate to the hair and no slip to give the blade. Soap without a lubricating component is worse than any cream, because it strips the skin oil that was helping.
Hair also needs roughly two to three minutes of contact with warm water before it is soft enough to cut at low pressure. Shaving straight out of a hot shower gets you that for free. Splashing your face twice does not. Product selection for reactive skin is covered in shaving with sensitive skin.
Cause two: a dull blade
A sharp blade severs hair with almost no force. A dull one bends the hair, then tears it, and needs you to push to get either.
Cartridges give five to seven shaves before they start tugging. Double-edge blades give three to seven, depending on the blade and your beard. The signal is not a date: it is the first stroke failing to clear a patch that it used to clear.
Blade preference is more personal than razor preference, and the difference between two brands of double-edge blade in the same razor is larger than most people expect. A sampler pack settles it in a few weeks. The razors themselves are covered in best safety razor.
Cause three: pressure
The correct pressure for a safety razor is the weight of the razor. For a cartridge it is only slightly more, enough to keep the head in contact.
Pressure feels productive because it does remove more, but what it removes is skin as well as hair. It also compresses the skin, so the blade cuts below the resting surface, which is the mechanism behind razor bumps rather than burn. Pressing hard is the one error that causes both conditions at once.
Angle is the reason people press. Held too flat, a safety razor skips and does not cut, so the instinct is to push. Roughly thirty degrees between blade and skin is where it cuts on its own.
Cause four: alcohol afterwards
A traditional aftershave splash is largely ethanol, and it is meeting skin that has just lost part of its protective layer. The sting is not the product working. It is a solvent on exposed skin, and alcohol dries the surface further while it evaporates.
Use a balm or a light unscented lotion instead. If you like a splash, an alcohol-free witch hazel or a glycerin-based tonic gives you the ritual without the solvent. Why the sting happens chemically is covered in why does aftershave burn.
Grain, passes and the neck
The neck accounts for most razor burn, for three reasons. Skin there is thinner, it is not stretched over bone, and neck hair usually grows in swirls rather than straight down.
Shaving straight down across sideways-growing neck hair means the blade is scraping rather than cutting, and the natural response is more pressure and more passes. Map your neck grain specifically, because it almost never matches your jaw, using the grain map tool.
Then reduce the passes. With the grain, then across the grain, and stop. Re-lather between every pass so the second one has as much glide as the first. A third pass against the grain over an already worked neck is the single most reliable way to produce burn.
What to do when it has already happened
Rinse with cool water, pat dry rather than rub, and apply something plain and unscented. Then leave it alone for a day or two if you possibly can.
The thing not to do is shave over it the next morning with the same setup. Shaving irritated skin removes more of an already-thin surface layer, and the redness compounds day after day until people conclude their skin is simply sensitive.
If you must shave, take one pass with the grain using a fresh blade and skip everything scented afterwards.
The short version
Razor burn is surface damage from friction, and every fix reduces friction. Two to three minutes of warm water, a lather wet enough to glide, a blade sharp enough to cut without pressure, the weight of the razor and nothing more, two passes maximum, and no alcohol afterwards. Fix the neck grain first, because that is where most of it happens.