There are many different font file formats, new and old, old being from older systems of computers so as mac os classic and Linux. Most people user fonts files that are accustomed to Windows and Mac OS. The below formats if what we recommend using when you upload your own fonts:
TrueType/TTF: This format is based for Windows, but can also be used by Mac users. It is the most commonly used font file type for its flexibility and all round good quality. If you have the option, we recommend this format for its universal usage.
OpenType/OTF: This format is based for Mac, typically when you download a font from a website, they rather supply you with both ttf and otf for windows and mac or they will ask which you would like.
There isn't any particular difference to the user, but to the editor of the fonts, it matters as when they're making the fonts they have different output settings.
Note: When downloading or using fonts, ensure that the font is legally allowed to be used through licensing. Some fonts allow for all kinds of usage but some require a purchase of the font which comes with a license to use.