It began--well, it began over 250 years ago, but most recently it began when a black high-school student asked for permission to sit under the "Whites Only" shade tree at the high school in Jena, Louisiana. The next morning saw nooses hanging from the tree (in the school colors). If you don't know what happened after that, you should.
You can find a summary here where Xicanopwr provides lots of useful links, with regular updates here.
Another good summary with updates here.