The Tumbleweed distribution is a pure rolling release version of openSUSE containing the latest stable versions of all software instead of relying on rigid periodic release cycles. The project does this for users that want the newest stable software.
Tumbleweed is based on Factory, openSUSE's main development codebase. Tumbleweed is updated once Factory's bleeding edge software has been integrated, stabilized and tested. Tumbleweed contains the latest stable applications and is ready and reliable for daily use.
This idea has been discussed in mailing lists for a long time and was conceived into action by Greg Kroah-Hartman, originally as an 'add-on' set of rolling updates which could be layered on top of a regular openSUSE release. On November 4th 2014 the Tumbleweed rolling release and Factory rolling release merged, leaving the single openSUSE Tumbleweed rolling release we have today