AR15 Stocks are either fixed or collapsible.  The same is true for AR10/LR308 Stocks.  The Mil-Spec A2 Fixed Stock results in an AR15 or AR10/LR308 with a more traditional M16 look and feel.  The collapsable stock was standard issue on an M4 and has the advantage of being adjustable to fit an individual shooter, as well as shortening the weapon for transportation.