The font typeface, styles, and even the size for text in your presentation are dictated by the active Theme or Theme Fonts set. You can certainly override these defaults, and select another font typeface and also change the font size. Beyond changing the typeface and size, you can also make a certain word or phrase stand apart by formatting it bold, italic, underlined, etc. Typically, text is formatted bold to attract attention; and italicized to add emphasis or to mark foreign words. You can also underline, and even strike through any selected text. All these format choices such as bold, italics, underline, strikethrough, etc. are part of a larger font formatting feature called Font Styles.
Learn about applying various font styles in PowerPoint 2011 for Mac.

