A search engine results page (SERP) is the actual page that a search engine like Google displays following a search query. Google is constantly changing the layout of their SERPs in order to make it easier and faster for people to find information in Google, and to maximize the number of clicks on the paid ads.
The two big components of a SERP include free/organic results, and paid results. If you are working on an SEO campaign, you will be primarily focused on the organic results, and if you are working on a PPC campaign, you will focus on the paid results. Studies show that your best bet is to appear in both paid and free listings for the same keywords, but that will not always be the possible.
Oftentimes, you will hear people refer to organic results as snippets. This is because the organic result on a SERP is drawn from the metadata snippets on your website. In contrast, the paid listings are drawn precisely from the ads that you create, which gives you more control over their appearance. If you want to optimize your organic results on a SERP, you need to re-write the html on your website — this process is a part of on-site SEO.