Google AdSense is nowadays one of the most popular ways to earn money online. It's easy to set it up and easy to use. Well, it actually works for you. Basically it works like that. Google offers you to put other websites ad's to your websites and when someone on your website clicks on the ad you earn some cents from it. If you have about 1000 visitors a day on your website you can easily earn about $5-$10 just from those ad clicks. The amount you earn from a lick depends on the value of ad, every different ad niche makes you different amount of money. The rarer the niche is, the more you probably earn from a click on this niche ad.
AdSense is kind of easy to setup. You have to login to your Google account, open AdSense homepage, choose the size of the ad, colors of the ad and then you'll get the HTML code that you can put on your homepage or blog. You can use AdSense in every kind of website, it doesn't matter if it's Wordpress based, Blogger or something different.
It's problably hard to make a living with Google AdSense only, but it certainly is a great way to earn some extra money for example Christmas or different holiday gifts.