Our font are available as .OTF and/or .TTF files. These are installable font files. We have written a tutorial in which we explain what these formats are and the differences between them.
Simply put, .OTF is a modern version of .TTF. However, either format will work the same on most devices. We typically recommend using the .OTF version as it has some advanced features.
If you're looking for fonts in a Webfont format, such as .WOFF, please see our Webfont help article.
Some designers include bonus files with their Fonts. You can often view which file formats these come in by looking at the product page and/or preview images.
If you have any more questions, feel free to reach out to us at firstname.lastname@example.org.