Here is a detailed article written by the driver of said Jeep:
http://www.wired.com/2015/07/hackers-remotely-kill-jeep-highway/
All of this is possible only because Chrysler, like practically all carmakers, is doing its best to turn the modern automobile into a smartphone. Uconnect, an Internet-connected computer feature in hundreds of thousands of Fiat Chrysler cars, SUVs, and trucks, controls the vehicle’s entertainment and navigation, enables phone calls, and even offers a Wi-Fi hot spot. And thanks to one vulnerable element, which Miller and Valasek won’t identify until their Black Hat talk, Uconnect’s cellular connection also lets anyone who knows the car’s IP address gain access from anywhere in the country. “From an attacker’s perspective, it’s a super nice vulnerability,” Miller says.