Desktop fonts are generally licensed perpetually, based on the number of users. These include the formats OT (OpenType), TT (TrueType), and OTTT (OpenType containing TrueType outlines). Most OpenType fonts contain PostScript outlines, which while technically no more compatible than TrueType, remain the most readily accepted standard by print houses and service bureaus in the graphic arts industry.
The webfonts that FontShop carries are the kind you host yourself*. These are usually licensed for a single website, and based on the estimated number of pageviews over a specified period of time. The industry standard formats for webfonts are WOFF (Web Open Font Format), and EOT (Embedded OpenType). These are marked simply 'Web' on the FontShop site.
*Webfonts from our house brand, FontFont, include a code upon purchase redeemable by Typekit, in case you'd like them to host the webfonts for you.
Embeddable fonts for eBooks, apps, games, and editable PDFs, including FontFont's App+ fonts are licensed based on a given period of time. TrueType is the standard format for embedded fonts.