Arial:
Availability: Widely available. It is probably the most common sans serif font. It is the default font for Windows, and it first shipped as a standard font with Windows 3.1.
Readability On Screen: Not the worse but definitely not the best, especially at small sizes, when it becomes too narrow and the spacing between characters too small.
Character: Has a streamlined, modern look but is also plain and boring. *For Mac users, the equivalent of Arial is Helvetica.
Times New Roman:
Availability: Widely available. It is probably the most common serif font. It is the default font for web browsers. It was first shipped as a standard font with Windows 3.1.
Readability On Screen: Acceptable for font sizes of 12pt. and up, but terrible for smaller sizes.
Character: Serious, formal and old fashioned. *For Mac users, the equivalent of Times New Roman is Times.
Verdana:
Availability: A widely available sans serif font, Verdana was first shipped with Internet Explorer version 3, when the exponential growth of the Internet demanded a new font that was easy to read on the screen.
Readability On Screen: Exceptional. It's wide body makes it the clearest font for on-screen reading, even at small sizes.
Character: Modern, friendly and professional.
Georgia:
Avaliability: Good. It is a serif font introduced by Microsoft with Internet Explorer version 4, when the need for a serif font with much better readability than Times New Roman became evident.
Readibility On Screen: Very good. It is the best serif font for on-line reading, since it was specifically designed for that purpose.
Character: Modern, friendly and professional.
Microsoft has also popularized two more fonts: Comic Sans Serif and Trebuchet.