SEA in a way is the opposite of SEO, as a paid placement tool. However, advertising and organic research are not mutually exclusive but could be integrated to achieve the best possible results.
SEA means precisely Search Engine Advertising, which is Advertising for Search Engines, and is a SEM (Search Engine Marketing) tool as well as SEO.
In practice, key keywords or keyword groups need to be studied to capture the right visitors from the search engine and buy visibility for those terms. You buy clicks (PPC, Pay per Clicks), which are all the more useful as you’ve done a good campaign setting job.
Google Adwords is not the only platform for making SEA, but the enormous spread of Google, especially in Western countries, has made Adwords become a true synonym for “SEA” for many.
In any case, SEA is paid advertising that is included in the search engine results (above, below or even on the side of the organic ones).
While organic placement requires a lot of months of work and a very limited power on effective yield, SEA results are quick and controllable.
The main job is the study of referring audiences and keywords, the campaign is properly set up and you start to gain tremendous visibility by paying every click you receive.
Also, you should also consider that campaigns on Google Adwords are less subject to changes to the search engine.
Google may change its position to the ads, causing major breakthroughs in the results, but these changes are certainly more manageable and clear than those made in the algorithm that affects work on the organic.