http://creativecommons.org/licenses/by-sa/2.0/

Note: OSM is in the process of changing to a new license ODbL: http://www.opendatacommons.org/licenses/odbl/. This license is under construction. It has about the same intent as CC-by-SA but without the negative side effects (hopefully). So, if possible, try to get permission to use the data under the CC-by-SA and ODbl license.