Democratic incumbent: Joe Biden
Republican candidate: Christine O'Donnell
I described Biden's background here, when he was about to be named as Barack Obama's running mate.
O'Donnell, 39, has worked as a political commentator on television and as a media consultant. This is apparently her first bid for elective office.
Polls listed in Real Clear Politics show a commanding lead for Biden. Oddly, that page describes Biden as being unopposed, even though it shows O'Donnell's poll numbers. However, given the size of those numbers, it seems as though the Democratic incumbent is, in effect, unopposed.
I wrote here about the implications of Biden's running for senator and vice president on the same day.
Biden was junior senator to Republican William Roth for 28 years. Democrat Tom Carper, who was then governor, defeated Roth in 2000, and was reelected in 2006.