title: What is a .a (as libcrypto.a) file?
body: <p>What exactly are <code>.a</code> (e.g. libcrypto.a) files and what do they consists of? Is it <code>.m</code> or <code>.o</code> or <code>.h</code> or <code>.c</code> files?</p>
  
  <p>How does IOS SDK/Xcode understands them? And if we include the <code>.a</code> file in our xcode project, do we need to copy the correspondng sources file too?</p>