Cold brew is coffee steeped in cold or room-temperature water for 12 to 24 hours. It is not the same as iced coffee — iced coffee is hot-brewed and then chilled — and the difference is chemistry, not temperature. Without heat, the acids, aromatic oils, and bitter compounds that define a hot cup extract much more slowly and unevenly, so what you end up drinking is a coffee stripped of most acidity and bitterness, dominated by sweetness, body, and chocolate or nut notes.
The method has old roots — the Dutch were brewing a version in the 1600s, the Japanese use a similar technique called "Kyoto-style" slow drip — but its modern popularity dates to specialty cafés in the 2000s, where Stumptown and Blue Bottle turned cold brew concentrate into a staple summer menu item.
A reliable recipe for a jar at home: 100 g of coarsely ground coffee to 1000 g of cold filtered water (1:10 as concentrate), steep for 16 hours in the fridge, then filter through a paper filter or fine cloth. Dilute 1:1 with water or milk before drinking. Because there is no heat to mask green or underdeveloped flavours, cold brew rewards clean, well-roasted coffee and a grind on the coarser side — think French-press coarse. Finer grinds over-extract and turn the cup bitter even at low temperature.