addlinksforgoogle.user.js

function engine(name, link,img) {
    this.link = link;
    this.img = img;
	this.name = name;
}

var form = document.forms.namedItem("gs");
var input = form.elements.namedItem("q");
var eq = encodeURI(input.value);

var engines = new Array();
engines.push(new engine("Google for Japanese",
	"http://www.google.com/search?q=%1&ie=UTF-8&lr=lang_ja",
	"http://www.google.com/favicon.ico"));
engines.push(new engine("Google for English",
	"http://www.google.com/search?q=%1&ie=UTF-8&lr=lang_en",
	"http://www.google.com/favicon.ico"));
engines.push(new engine("Twitter Search",
	"http://search.twitter.com/search?q=%1",
	"http://a1.twimg.com/a/1273181936/images/favicon.ico"));
engines.push(new engine("Yahoo for Japanese",
	"http://search.yahoo.co.jp/search?p=%1&ei=UTF-8",
	"http://www.yahoo.com/favicon.ico"));
engines.push(new engine("Yahoo for English",
	"http://search.yahoo.com/search?p=%1&ei=UTF-8",
	"http://www.yahoo.com/favicon.ico"));
engines.push(new engine("Bing",
	"http://www.bing.com/search?q=%1",
	"http://www.bing.com/s/wlflag.ico"));
engines.push(new engine("Hatena Bookmark Search",
	"http://b.hatena.ne.jp/search?q=%1",
	"http://b.hatena.ne.jp/favicon.ico"));

var header = document.evaluate( 'id("leftnav")/div[2]', document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null);
header = header.singleNodeValue.firstChild.nextSibling;

if (header != null){
	var html = '';
	var cur;
	while(cur = engines.shift()){
		var tmp_html = cur.link;
		tmp_html = tmp_html.replace(/%1/g, eq);
		html += '<li class="tbou"><a href="' + tmp_html + '">';
		if(cur.img){
			html += '<img width=16 height=16 border=0 src="' + cur.img + '">';
		}
		html += cur.name + '</a></li>';
	};

	html = '<li><ul class="tbt tbpd">' + html + '</ul></li>';
    var new_element = document.createElement("ul");
	new_element.className = "med";
	new_element.id = "tbd";
    new_element.innerHTML = html;
    header.parentNode.insertBefore(new_element, header);
};


// ==UserScript==
// @name          Add Search Links for Google
// @namespace     http://d.hatena.ne.jp/stanaka/
// @description	  Add links of other search engines to sidebar of Google search results
// @include       http://www.google.*/search?*
// ==/UserScript==

What exactly does this script do?