I'm using Windows XP, and while going through and organizing my font library, I noticed that for many fonts, I have different formats - True Type (TT), Type 1 (a), and Open Type (o) of the same font. What's the difference and do I need all these different formats of the same font?

OpenType is the most flexibile format. It can contain both Type1 and True Type splines. Generally, it depends of the software in which you will be using the font. OT offers some advanced typographic features which were not available in Type1 and True Type formats (Alternative characters, Stylistic sets various supstitutions etc.).

Thanks for the info!

I don't know how or why, but some TrueType fonts also feature similar functionality.

Junicode comes to mind.