Technically, it's an amendment to President Johnson's landmark 1965 Executive Order that similarly outlawed discrimination on the basis of race, creed, color or national origin – and now includes sexual orientation and gender identity as protected categories.
The President acted because the Employment Non-Discrimination Act (ENDA) that he's championed to ban discrimination on a national basis has failed to make it through Congress. While the Executive Order got subsumed in national media awash in geopolitical events recently, it has the potential to impact up to a fifth of the U.S. workforce, and even some non-federal contractors as well.