Search engine optimization (SEO) is the practice of increasing the quantity and quality of traffic to your website through organic search engine results.
To understand what SEO really means, let's break that sentence down and look at the parts:
Quality of traffic.
You can attract all the visitors in the world, but if they're coming to your site because Google tells them you're a resource for Apple computers when really you're a farmer selling apples, that is not quality traffic. Instead you want to attract visitors who are genuinely interested in products that you offer.
Quantity of traffic.
Once you have the right people clicking through from those search engine results pages (SERPs), more traffic is better.
Ads make up a significant portion of many SERPs. Organic traffic is any traffic that you don't have to pay for.
How SEO works
You might think of a search engine as a website you visit to type (or speak) a question into a box and Google, Yahoo!, Bing, or whatever search engine you're using magically replies with a long list of links to webpages that could potentially answer your question.
That's true. But have you ever stopped to consider what's behind those magical lists of links?
Here's how it works: Google (or any search engine you're using) has a crawler that goes out and gathers information about all the content they can find on the Internet. The crawlers bring all those 1s and 0s back to the search engine to build an index. That index is then fed through an algorithm that tries to match all that data with your query.