OTF or TTF?
Use OTF font files if you have them. Otherwise, TTF is fine.
“OTF” stands for “OpenType Font”. It is a more modern format than TTF (“TrueType Font”).
OTF fonts usually provide additional controls for typographic features, such as ligatures and contextual alternates. On the web, you can enable or disable those features using the font-feature-settings CSS property.
I wrote this post after the umpteenth time that I downloaded font files and forgot which format I wanted to use. Hopefully, I never forget again! At the very least, I know where to find this information again — I hope it helps you, too!