来源于： Learning jQuery, 4th Edition
What jQuery does:
1. Access elements in a document;
$('div.content').find('p');
2. Modify the appearance of a web page;
$('ul>li:first').addClass('active');
3. Alert the content of a document;
$('#container').append('<a href="more.html">more</a>');
4. Respond to a user's interaction;
$('button.show-details').click(function(){
$('div.details').show();
});
5. Animate changes being made to a document;
$('div.details').slideDown();
6. Retrieve information from a server without refreshing a page;
$('div.details').load('more.html #content');
7. Simplify common JavaScript tasks:
$.each(obj,function(key,value){
total +=value;
});