Style mostly. Black Metal often employs more theatricality than Death Metal - corpse paint, leather outfits, grandiose set design in music videos...etc. Black Metal bands tend to focus their music on more supernatural and religious themes (mostly anti-Christianity, especially when it comes to bands from places like Norway). It also differs in vocal style, with Black Metal using higher pitched, screeching vocals, as opposed to the deeper, growling vocals used by Death Metal bands.
Musically, black metal is generally quite a bit more minimalistic than death metal.
A lot of death metal bands place a premium on "technicality." Their songs feature a number of tempo and melody changes, and can be quite complex, sounding more like a collage of songs rather than a single song.
Black metal songs are more simple and straightforward. As Wikipedia puts it, "many black metal songs contain lengthy and repetitive instrumental sections."
Death metal bands like clean production, to show off the aforementioned technicality. Black metal bands prefer "dirty," lo-fi production: the joke is that a truly "kult" black metal song should sound like it was recorded in a forest.
Death metal bands tune their guitars down for a deeper, heavier sound, and to match the guttural vocal style. Black metal bands favour higher-pitched guitar-playing, again to match the shrieking and screaming from the vocalist. The bass guitar plays a much less important role in black metal than in death metal.
Finally, if you like a death metal song, you say it's "brutal." If you like a black metal song, it's "grim".
There is a certain amount of crossover between the two. Death metal bands who go in for black-metal type lyrics, like Deicide, Behemoth and Akercocke, are called "blackened death metal." Black metal bands also sing about death, but they prefer to sing about war and suicide rather than murder and torture.
They have vocals in death metal?
