Both African rhino species are grey, and individuals take on the colour of whatever mud they last wallowed in. The useful distinction is the mouth. The white rhino has a broad, straight upper lip roughly the width of a spade, suited to cropping short grass; the black rhino has a prehensile, pointed upper lip for pulling twigs and leaves off woody plants.
The difference cascades through the animals' lives. White rhinos graze in open grassland, are relatively placid, and are often found in loose groups. Black rhinos browse in thicket, are solitary, and are markedly more aggressive when disturbed. Head carriage differs too, with the white rhino holding its head low and the black rhino higher.
The popular explanation that white is a mistranslation of an Afrikaans or Dutch word for wide is widely repeated but not securely documented; the English term appears before the supposed borrowing is attested. The name may simply be a contrast coined after black rhino was already in use.