Oh, the joys of file formats. If you’re new to the gloriously confusing world of design, you have probably already noticed the ridiculous amount of file types thrown about the place.
There are many different formats of fonts that can be installed, however, the most common are .ttf (TrueType) and .otf (OpenType). With OpenType being the newer, more favored format. They will both work on Macintosh, Windows, and Linux machines.