Web fonts are hosted by an external server or service such as Google Web Fonts, Adobe Typekit, or Monotype Inc. Fonts.com. These web fonts allow designers to pull typefaces from a server to display those typefaces on their own websites.
Without Web Fonts, a user's computer may not display the right font because their computer lacks the proper file. It is very important to declare and use web fonts, or a user's computer will determine a replacement automatically.
Web Fonts vs. @font-face
@font-face is a css feature that allows you to use fonts that exist on your computer as a web font. It is advisible that you only use @font-face when you are certain your typeface has the correct copyrights that allow it to be used as a web font.
Web Fonts are typefaces that have been optimized and cleared for use on websites.