Complex sentence: In June 2003 Wikipedia was transferred to a nascent non-profit organization, the Wikimedia Foundation, which was formed as a charitable institution to supervise Wikipedia and its associated wiki-based sites.
Split into simpler sentences: In June 2003, the property of Wikipedia was formally given over to the nascent non-profit organization, the Wikimedia Foundation. The Wikimedia Foundation was formed to serve as a charitable institution with a mission of supervising Wikipedia and multiple other associated wiki-based sites.