There's more information on Ripple contracts here: https://ripple.com/wiki/User:Justmoon/Contracts:_Overview

BTW, while the Ripple network supports the ability to establish trust lines (also known as IOUs) between users, the cryptocurrency (XRP) is separate from that.

In fact, XRP is the only currency on the Ripple network that does't have counter-party risk and doesn't require a trust relationship to transact in.