Search engine optimization (SEO) is an process of optimizing your website and its content like text and images etc, to achieve the best possible ranking in Google,s search engine results page without any paid advertisement.
A well-optimized webpage will increase traffic for your site, which will ultimately increase your ranking in SERP leading to increase in conversions and sales.
Google AdWords (also known as pay-per-click) is a platform run by Google that allows companies to advertise their website in search results when a user searches for a specific keyword. Advertisers "pay per click" on their advertisement. These ads show on the top of the search results page, as well as on the side of the search results depending on the search being initiated.