Organic SEO is process of building the ranking of your website through building linkbacks, citations and on-site optimization using the requirements of Google and other Search Engines. An Organic listing of your website link is below any Paid Ads and is not charged a fee when clicked on by visitors. The position of your listing is dependent on your rank and the rank of your competitors website listing.
Organic listings are more permanent and are usually much less costly than PPC Ads. With PPC Ads, your listing will dissapear the moment your Ads are turned off and can cost thousands to maintain. However PPC Ads can have instant results vs Organic Rank takes a few months to achieve first page listing.