With the release of Microsoft Office 2007, Cambria (for headings) and Calibri (for body text) have replaced Times New Roman as the default font. The new fonts were made with readability in mind.

The new template uses fonts that were designed with readability in mind. According to a blogthat was active during Word 2007 development, “the new fonts used in Word are based on the ClearType technology that provides a crisper, more easily read display of the fonts on most modern monitors.” Calibri is the new default font for body text, and “to complement the look of Calibri, Cambria was selected for use in headings.”

http://office.microsoft.com/en-us/word/HA102310271033.aspx

Unfortunately these new fonts are not accepted by some teachers. When writing an essay, thesis, or any formal document, and you are unsure which fonts are allowed, use Times New Roman. You might wanna’ call it the fail-safe of fonts.