Well it's exactly what it sounds like. Normally hot water is used to extract the flavour from ground coffee beans quickly. Cold brewed coffee uses cold water to gently extract the coffee from the grounds over a much longer period of time (12 hours usually instead of a minute or so for hot extracted coffee).
This results in a mild smooth flavour compared to espresso or filter coffee but also a high caffeine level. Cold brew coffee is still full of flavour but with much less bitterness so it is often served black on ice.
If a creamy mouthfeel or texture is desired the cold brew can be infused with nitrogen gas and then pushed through small holes under pressure. Doing this essentially "whips" the coffee and it pours creamy and white with a dense foam on top like a cappuccino without using any form of milk, dairy or otherwise. Pouring using Nitro also creates attractive cascading bubbles that are visible if served in a glass (it looks similar to a Guinness stout beer!).