.ttf - True Type Font Files
Usually downloadable fonts are packed inside .SIT or .ZIP files. .SIT files were popular in Mac OS 9 days, and today ZIP files are preferred.
TTF files may in rare cases be packaged inside of .EXE files, which is unpractical for Mac users.
On some EXE files you may "strip off" the EXE stuff with File Juicer. Check the "Zip, CAB" check box in File Juicers preferences and drop the exe file onto File Juicer.
Inside the EXE file could be a .ZIP or a .RAR file with which is a compressed file type which can be expanded with Stuffit Expander or RAR Expander.
This is not special for font files and any file type can be wrapped inside an EXE file this way.