This excerpt taken from the YHOO 10-K filed Mar 3, 2006.
Yahoo! Search on Mobile provides users two ways to complete searches on their mobile devices using a web browser or text messaging. In 2005, we launched text messaging search, which enables users to use text messaging to retrieve data from the Internet using any mobile phone that supports text messaging, while the enhanced web browser search functionality allows users to access Yahoo! Search on a wider variety of devices in addition to mobile phones.