Calibri is a sans-serif font for Microsoft Word. It was designed by Lucas de Groot for the Microsoft Office database system, and was considered a font easier to read than the standard Times New Roman. As the sans-serif fonts gained popularity for their readibility, Calibri, as well as its font family, became more widespread, and eventually Microsoft Office 2007 replaced the Times New Roman 12-pt font as the starting font with the Calibri (Body) 11-pt font, making Word documents more comprehendable.
Image of the font:
http://upload.wikimedia.org/wikipedia/commons/4/46/Calibri_example.svg