When you get billed for each visitor through search engine, the business model is known as Pay per Click (PPC). Pay per Click results usually appear as "sponsored link" above or besides the organic search engine rankings.

Organic search engine results are displayed as normal URL's on SERPs (Search Engine Result Pages).

Although, both i.e. the results from organic optimization and PPC campaign are displayed on the same page, there are numerous differences between them:

• PPC results can start showing up within 48 hours as compared to organic search results which may take ~6 months or more.
  
  
• With PPC each visit is being paid for! With organic search engine rankings, there is certainly an optimization costs but then that has the potential of attracting innumerous visitors without much change in the optimization costs.
  
• PPC gives much greater control over what advertisement you wish to display, who you wish to display your URL to and when. While with SEO, it is much more dependent upon the search engines and their algorithms.

Advantages and Disadvantages:

Organic Search Engine Optimization
Organic search engine rankings are considered more cost effective as compared to a Pay per click program. According to a study conducted in 2005-06, 70% of the search engine visitors reported clicks on organic search engine rankings and only rest 30% clicked on the Pay per click results.
One of the drawbacks of organic search engine rankings is the duration required to get your website within the top listings. Also the ever changing search engine ranking algorithms revolves a fear of dropped search engine rankings and de-listing.

Pay per Click
The major benefit of Pay per click program is the immediate listings and the complete control of listings in the hand of the advertiser. The bid under the google adwords program begins from $0.01 and Yahoo Overture starts from $0.10 which can further go up to any limits.
The only drawback with the Pay per click program is the cost involved in the same. Achieving top positions for some of the search phrases under the sponsored results can cost thousands of dollars a day.

Summary
With the increasing use of Internet everyday and emergence of the Internet as one of the most powerful media, PPC and organic SEO offer different options. However, small businesses must consider both the activities and perhaps, perform both ensuring a fine blend for enhanced ROI.