Most likely you've heard this by now, but here is the link to the NY Times report should you care to read more about Biden's reasoning and the ripples resulting from his announcement. In brief:
Vice President Joseph R. Biden Jr. announced on Wednesday that he would not run for president, ending a period of remarkably public agonizing and clearing away one of the biggest potential obstacles to Hillary Rodham Clinton’s path to the Democratic nomination in 2016.
In a hurriedly arranged speech in the White House Rose Garden with President Obama at his side, Mr. Biden said that he and his family had overcome their grief at the death of his elder son enough to commit themselves to the rigors of a campaign. But with just days until the first filing deadlines, he said he had concluded that it was simply too late.
"Unfortunately, I believe we’re out of time, the time necessary to mount a winning campaign for the nomination," Mr. Biden said. "But while I will not be a candidate, I will not be silent."