Don't start new threads for this question. Just search on Google and you'll find 100s of 1000s results
Adsense is a program by Google. Through the Google Adwords program advertisers pay/bid on certain keywords to get their ads placed on various sites or on pages of search results. You place the ads on your sites(s) and if visitors click the ads, Google will share a portion of the revenue they received from the advertiser with you!
You can find many information about adsense on the internet just by searching adsense in the Google. And you can learn everything about it in the DP.
All you have to do is have a blog or website and add the html code that Google gives you when you have an account with them to your blog or website and if you have good traffic and they click your ads you make money.
better read it again since there are many things more that you need to learn by yourself or you can make money with Adsense.
Adsense is the other side of Google's Adwords. That is Google Adwords are the ads that people want advertise and Adsense are were the ads appear. Your Website is Internet real estate. The more your website is visited the more valuable the real estate. Just like a piece of real estate you can sell bits off. In this case it is the same as you putting up an advertising billboard on your property. You rent this to a company; in this case Google Adsense and they put their ads on it.