Search Engine Marketing

Search engine marketing(SEM) is a tool of online marketing which promote company’s website on the internet to attract more internet users to visit the website. Search engine optimization (SEO) is part of SEM.

Google is one of the biggest SEM in the world, others such as Yahoo, Baidu, Microsoft etc. Take Google for example, there are 2 kinds of research: Paid listings and Organic (or natural) listings.

Paid listings are those which need money to pay for their links and usually used is pay per click (PPC). Here are some advantages of paid listings:
-Listing is guaranteed and instant
-Flexible, allows for trial-and-error
-Search engine provides reporting
-Context/Geo Targeted (show the pacific area)
But also have disadvantages:
-It's very expensive to maintain.
-It also shows users are 80% less likely to click
-Competitors with deeper pockets can outbid you

For me, usually when I search for something, I always click the organic listings.
Organic listings are usually free. The Advantages of that:
-There is low cost means no money to search engines
-It shows users are 80% more likely to click
-High, positive can last long into future
-Perceived as a quality endorsement
Disadvantages:
-There is time required before results appear
-Difficult for high competitive phrases
-No guarantees

You can also go through http://www.yoursite.com/ to check how popularity on your own site.

Reference:

http://www.hubspot.com/organic-vs-paid-search/#ixzz0mOZAXIs3