A bookmarklet is an applet, a small computer application, stored as the URL of a bookmark in a web browser or as a hyperlink on a web page. They are frequently JavaScript commands to extend the browser's functionality.
Every functionality you get with a bookmarklet can be duplicated using a browser's console and a little time. Bookmarklets merely simplify the process -- packaging up the code that implements some functionality inside a neat little button.
Categories of bookmarklets according to Nettuts+

• Ones that pass on data. Bookmarklets that submit a page to a service fall under this category. Bookmarklets dealing with social media, dictionary lookups, searches all fall under this category. e.g. submit text to a dictionary.
• Ones that obtain information and/or modify the current page. e.g. set a page's background color.
• Ones that works behind the scenes. e.g. erase any cookies of the current site.

Examples
Send current page to Reddit:

javascript:location.href='http://www.reddit.com/submit?url='+encodeURIComponent(location.href)
+'&title='+encodeURIComponent(document.title)

Generate href link for current page

javascript:function htmlEscape(s){
  s=s.replace(/&/g,'&');
  s=s.replace(/>/g,'>');
  s=s.replace(/</g,'&lt;');
 return s;
 } 
 function linkEscape(s){
  s=s.replace(/&/g,'&amp;');
  s=s.replace(/"/,'&quot;');
  return s
  }
  h = '<a href="' + linkEscape(location.href) + '">
  ' + htmlEscape(document.title) + '</a>';
 with(window.open().document){write(h+'<form name=f>
 <textarea  name=a rows=5 cols=80 wrap=hard>'+htmlEscape(h)+'</textarea></form>');
 close();
 f.a.select();
 } void 0

javascript: (function () {
 var z=[],N,href,tc,j;
 function r(N) {
  if(N.href) z.push(N);
   var C=N.childNodes,i;
   for(i=0;i<C.length;++i)
     r(C[i]);
   }
 r(document.body);
 for(j in z) {
  N=z[j];
  href=document.createTextNode(' [-- '+N.href+' --]');
  tc=document.createElement('span');
  tc.style.color='green';
  tc.style.background='%23FFEE99';
  tc.appendChild(href);
  N.parentNode.insertBefore(tc,N.nextSibling);
  N.parentNode.insertBefore(document.createTextNode(' '),tc);
  }
 })();

Installing a bookmarklet:
Desktop computers: Drag it to your Bookmark bar.
Dolphin browser on android see dolphin.