CSS class (disable-selection defined in example code)
unselectable="on" attribute (it is required by IE < 10 and Opera < 15)
onselectstart="return false" (it is additional solution for Internet Explorer web browsers family)

Notes:

some browsers still don't support user-select property (Can I use link here), so it is necessary to use prefixes for full capability.
in CSS3, user-select property is not part of the standard - it has working draft status for CSS4.

// ONLINE-RUNNER:browser;

<!doctype html>
<html>
<head>
  <style>

    .disable-selection {
        -webkit-touch-callout: none; /* iOS Safari                         */
          -webkit-user-select: none; /* Safari                             */
           -khtml-user-select: none; /* Konqueror HTML                     */
             -moz-user-select: none; /* Firefox in the past (old versions) */
              -ms-user-select: none; /* Internet Explorer (>=10) / Edge    */
                  user-select: none; /* Currently supported:               */
                                     /* Chrome, Opera and Firefox          */
    }

    div {
        margin: 10px;
        padding: 20px;
    }

  </style>
</head>
<body>
  <div class="disable-selection" 
       style="background: silver" 
       unselectable="on"
       onselectstart="return false">
     This text is not selectable ...
  </div>
  <div style="background: gold">
     This text is selectable ...
  </div>
</body>
</html>