Obama has realized that the Moderate Democrats would not support his Amnesty Immigration bill called "The Dream Act". He fought for two years to pass this between 2009 and 2010 when the Democrats controlled the majority of Congress and the White House.
So in August 2011, Obama signed an Executive order that provided the first 300,000 illegal residents with the same legal rights as granted under the Obama Dream Bill that Congress said no to. Therefore, Obama is making up laws and bypassing the Legislative Branch of Government. This happens to be a clear violation of the US Constitution.