A SERP is a results page generated by a search engine following a user's search. In other words, as an Internet user, you most certainly spend a lot of time on the SERPs! They display a list of results or websites seen as relevant by the algorithms. Some paid ads, if they are also relevant, may appear in a SERP. So, this is not a specific SEO term, although it is mostly made up of blue links (organic links).
If there is an infinite number of possible positions in a SERP, there are only 4 that are interesting for organic ranking: the position zero, (above all the results), the first, the second and the third positions. Beyond that, the click-through rate (CTR) is extremely low. Studies show that the first position’s CTR is around 31%, 24% for the 2nd, and 18% for the 3rd. Therefore, the Top 3 (and possibly position zero) is the top priority for any SEO who should also keep in mind that he/she faces competition from paid results.
The SERP’s different components
A results page is made up of different elements depending on the nature of the query. Some highly competitive queries generate very rich SERPs, while others generate only organic results (blue links). A SERP considered as “rich” can display a position zero (i.e. a result above “everything”), paid results (Google Shopping, Google Ads), images, and a knowledge graph (an information block located on the right). This same SERP can be furthermore enriched via rich snippets which qualify the results. Rich snippets can be stars for reviews or additional data such as cooking time, reading time, event date, etc. There are dozens of them available on schema.org. Finally, depending on the requests, you can find points of interest via Google Maps. This is often the case for local searches.
The SERP’s content depends on many criteria
The order in which results appear in the SERPs is determined by Google's algorithm, which covers several aspects. Although the Giant fiercely hides its ranking criteria, SEOs have determined that it evaluates websites thanks to hundreds of criteria that help gauge a particular web page’s relevance and value. Among the best known: the website's performance (loading time, etc.) and its mobile compatibility, its content, its internal linking, and the number of quality backlinks it receives.
The SERP may also differ depending on the user’s geolocation, his browser settings, and even the data center involved during the search (indeed, Google has several data centers). In other words, a SERP’s ranking is based on fixed but also variable criteria. That’s why you will not necessarily see the same results as a friend who lives 800km away from you.
Ranking in the SERPs is a very inaccurate science. It’s impossible to predict their evolution. However, by following Google's guidelines for good SEO practices, you will put all the chances on your side to appear in the Top 3.