Google Search Engine Results Page (SERP):
1. Paid Advertisements: Usually the first 3 listings on the top left hand-side of the search engine results page (SERP) are paid advertisements - also known as Search Engine Marketing (SEM) or pay-per-click (PPC) ads. These are listings that companies have set up through "Google Adwords".
When setting up these accounts, companies design the ad, choose keywords, and set a maximum bid for that keyword.
When there is a search for one of the selected keywords, Google runs an automated check to determine what ad should be placed in the top position (and subsequent positions) based matching of the search phrase, quality of ad and landing page, and top bid (among other elements).
Paid ads are only clicked on about 25% of the time; however, the company only actually pays for that ad placement when the searcher does click on the ad.
2. Google Plus (G+) influenced results: This part of the SERP is still new and does not show up consistently or often yet. I have only seen it a couple of times so far on my SERPs.
These results are influenced by the searcher's connections and interactions on Google Plus (G+) - Google's social media network. Overtime, it is expected that "social" results, especially from G+, will play an increasing (or at least more consistent) role in search engine results. It is for this reason that marketing consultants are encouraging their clients to create a G+ account.
3. Organic Search Results: The ads that show on the left hand-side of the page, under the paid ads, are the organic, or natural, search listings. There are generally 10+ organic listings on a page, and these ad spots on Google's "firsts" page of results are the most coveted and valuable ad spots that Google offers.
They are valuable because they are "clicked" on by searchers 75% of the time, and the company behind the ad doesn't pay for the ad space - ever. These ads have "earned" their position through high quality content, targeted search engine optimization (SEO), and time.
It is for these listings that you need to be producing original quality content and should undergo professional SEO.
4. Additional "Paid" Ads: The listings on the right hand-side of the page (under the G+ listing if it appears) are more "paid" ads. These ads did not qualify for the top 3 positions (on the left hand-side of the page) for this search term at this moment, but are ranked in positions 4, 5, 6, . . . They may rank lower because of a lower quality score or because of a lower bid price.
5. Filters: You can "click" on these filters to customize your search results or to find "related searches".
The Google Search Engine Results page is very influential in determining website traffic. A campaign to increase traffic can benefit from a combination of the production of frequent, high quality, original content; professional SEO; paid advertising; and social media marketing.