Microsoft Office 2007 introduces six new fonts, whose names all begin with the letter C: Calibri, Cambria, Candara, Consolas, Constantia, Corbel. I guess those names are meant to fit in with Windows Vista's theme of clear, confident, and connected. Here is a quick look at them. Select each font name for a sample of the letters.
Calibri is a general purpose sans serif font, like Arial and even more like Helvetica. It has rounded corners.
Cambria is a serif font, described by some as "sturdy", and has a less formal feel.
Candara is a sans serif that has flared ends on vertical stems, giving a concave look to the lines. It is reminescent of Optima/Zaft Humanist.
Consolas is a monospaced font, similar to Lucida Console.
Constantia is a serif font, with a more formal look with its smaller x-height.
Corbel is a sans serif font similar in function to Verdana -- good for screen reading.
Here are additional online articles regarding those fonts.