Google Adsense is still the most popular ads network.
At the beginning the cost per click was really impressive. It was at least 15 cents per click. Now it can be lower.
Adsense can be shown as text or pictures, sometimes even as videos.
It will show ads related to your website content, it is an easy way to have people and companies promoting products on your blog.
One of the problem is that more and more users will setup an ad blocker like AdBlock or AdBlock Plus to eliminate these ads. For the moment, it only concerns 9% of the users.
If you really want to make money with Google Adsense, you need to have at least 10k of visitors per day. And to discuss about very high paying subects.
Or you need to be in a niche, a sector where the concurrence is low.