OTF vs TTF fonts
OTF (Open Type Font) are cross-platform (Mac and PC) fonts. They typically have a smaller file size which allows for a quicker download. They support expanded characters which allows for alternate characters and ligatures.
TTF (True Type Font) are similar to OTF fonts but they require separate files for each instance of the font. For example, a separate .ttf file is needed for bold, italics, or bold italics.
For most intent and purposes, it will make little difference to use one or the other file type when downloading fonts from LD.