Cold Brew is a coffee made by mixing coffee grounds with water, over a long period of time. Typically cold brewed coffee takes 12-24 hours; at room temperature or lower.
Cold brewed coffee has less acidity and can tend to be more strongly caffeinated. It can be ideal as a base for iced coffee since no dilution will occur after being iced.
To make cold brew at home, here are some recipes:
Toddy makes a home appliance specifically for cold brew coffee or iced tea.