What's the difference between the file types of .otf and .ttf ?

The main difference is between the character right after the dot.

But seriously; the file extension is not that important. The differences between OpenType fonts with TrueType and CFF based outlines is explained here:
https://forum.high-logic.com/viewtopic.php?f=5&t=1619

haha, thanks!

They are distinguished primarily by their different outline formats and the contrasting approaches employed to rasterize those outlines. For designers, both amateur and professional, the main useful difference between OTF and TTF is in the advanced typesetting features. OTF features embellishments like ligatures and alternate characters—also known as glyphs—that exist to give designers more options to work with. In other words, OTF is indeed the “better” of the two due to the additional features and options, but for the average computer user, those differences don’t really matter. TTF stands for True Type Font , a relatively older font, while OTF stands for Open Type Font Open Type Font, which was based in part on the TrueType standard. TTF fonts are still so much more popular than OTF fonts.

http://net-informations.com/q/mis/otfttf.html