Ad sense is context based advertising by Google. It gets opportunity to web site owner to earn money throw publish ads from the Google adwords on his site. When some one clicks on these ads you get paid for these clicks.
For this process first you join adsense program throw your gmail id. When you make account on adsense you can start publishing ads. Ads format you can decide which type ad you want to publish like text or image or both. Adsense provides Code for these ads. Google provides java script code for these ads. You put this code on your web site.
Seo India point of view adsense is a good process to earn money. It is important you should choose an ad format which will fit into your site. Google provides relevant ads for your site.