Both are developer APIs that are used for browser plugins. NPAPI is a very old standard (the N stands for Netscape), whereas PPAPI is a modern successor developed by Google. Which one to use depends on the browser you are using. PPAPI has so far only been adopted by browsers that use Google’s Blink engine, which is Google Chrome, Chromium (the open-source base of Chrome) and the modern versions of Opera (2013 and newer). However, Google Chrome maintains its own Flash plugin, so you only really need the PPAPI for Chromium and Opera. Safari, Firefox and others still use NPAPI.