The Internet is a major advertising medium – one which continues to grow in importance every day. Most websites post ads of one sort or another. Types range from text ads, to banner ads, to the (often highly annoying) pop-up ads.
Like most other web development projects, posting ads to your site – and getting paid for having done so – is as complicated as you want to make it. You can spend hours every day tracking down ads that fit your site's topics and negotiating for compensation. Or, you can spend 15 minutes setting up a Google AdSense account and never worry about it again.
AdSense is an automated advertising system. The AdSense spiders crawl your site and analyze its content. Then the program posts ads that relate to your site. The idea is to post ads that are most likely to interest your visitors. If your site provides information about how to play the guitar, AdSense might post ads from guitar manufacturers, sheet music companies, or music teachers.
To set up AdSense ads, you need to sign up with Google. They will check your site and confirm that it has acceptable content (no pornography, gambling sites, sites promoting illegal activities, etc. are allowed) and then give you the HTML code to post on your site. Once you set it up, Google will pay you a small amount every time a visitor clicks on an ad posted on your site. Compensation is based on how much the advertiser bids for that ad.
You have the option to customize which ads appear on your site, and to block out ads you don't want; it's a good idea to keep an eye on your site's ads for the first few days or weeks and tweak your ad content filter as needed. I once visited a blog which disparaged the fast-food aspects of American culture and AdSense put up an ad for McDonalds franchises on the same page! Funny, but it probably didn't make that webmaster much money on ad click-throughs.
You can also choose whether to include text or image ads (or both) and you can customize the ads' appearance by tweaking color, layout, size etc. of the ads.
AdSense is not the be-all and end-all of web advertising. It's a great place to get started, since the program requires a minimal amount of time and no money to run on your website. And once you've gotten your feet wet, perhaps you'll be ready to start some ad campaigns of your own!