When you search any given keyword in a search engine, you will notice two different kinds of results. Paid results, and organic results. Here is a brief explanation of each using pictures to illustrate.

Paid search results are Ads offered by the search engines. The most common form of these is through a service called Adwords which is offered by Google.

Organic results are picked up from websites by the search engines, and ranked in a way that the search engine finds most appropriate.