Google Adsense is another 100% legitimate way to make some spare cash online. You could make anything from one hundred dollars to hundreds of thousands (not likely) depending on the amount of clicks your ads get!
You can see an example of Adsense on this page, look for the little ads to the right of this page. That's all Adsense is! You need to create a blog, or website and drive traffic to it by posting interesting, or relevant information to people.
Then, you sign up for Adsense here, get accepted (you need to have actual content on your blog/site) and post the HTML code to your blog or site! Simple as that!
To start earning revenue from it people must click your ads! So all you need to do is get traffic to your site/blog and Google does all the work creating ads relevant to that persons interests through their search history etc. (creepy huh?), they click the ads, you earn some spare cash!!