Hello, I would like to know or put the code of google AdWords remarketing ?
is there an extension for magento ?
To answer yor original question ther are many extensions in Magento Connect. Some Free some not so free! I use Fooman_GoogleAnalyticsPlus, Google_Tag_Manager_integration, and Tatvic_Google_Analytics_Enhanced_Ecommerce that makes Fooman a little redundant, but Tatvic is Beta so be careful, but I like it so far.
You should also look at creareseo and what I find better then AddThis is AddShoppers. I found the Addshoppers improved my page load time greatly and has more things to implement.
Pay attention to Page load Speed if you want a higher rating on Google and others. I paid for Full page cache by Amasty and WOW my page load is in millisecounds now, like from 0.8 to 0.002, When before it was 4 to 1.2 Secounds!!! Well worth the $200. I messed around with some page cache programs like Varnish but it is Difficult to manage, setup and prone to errors! An FPC seems to work better for me with standard PHP5 cache.