Search Engine Marketing (short: SEM) is a sub-area of online marketing and includes all measures to improve the visibility of a website within search results of a search engine. The measures can be paid ads within the framework of search engine advertising (SEA) as well as search engine optimization (SEO).
The term Search Engine Marketing is not always used unambiguously within marketing. Definitions can also be found in textbooks that see SEM as a generic term for SEO and SEA, while other sources use SEM as a synonym for search engine advertising or PPC advertising. 
In order to separate the terms Search Engine Marketing, SEO and SEA, it is necessary to break them down into their constituent parts:
Marketing per se is by definition not a means or a method, but an attitude of a company. Marketing itself includes all the necessary measures to optimally align a company with the needs of the market. In combination with search engines, marketing refers to measures that are necessary to achieve optimal reach and visibility within the search engines for a company (this can be an online shop or a web portal). Search Engine Marketing is in this sense an umbrella term for search engine advertising and search engine optimization.
This is a method that is used to achieve the maximum reach in relation to the budget used or the maximum ROI by means of paid advertising in search engines. SEA itself can be used through different channels or be part of a marketing campaign.
is also a method to improve the ranking of relevant keywords in search results lists. Optimizations affect not only OnPage factors, but also OffPage factors. The necessary tools for optimization can be divided into different sub-categories. However, the term search engine optimization is criticized today because it is too strongly concentrated on search engines and not on the user. Thus, SEO is not only a measure to achieve better rankings in SERPs, but also to improve the user experience. In this sense, SEO is sometimes associated with "Search Experiences Optimization".
SEM can fulfill various objectives. As a rule, they are similar to the usual target definitions in marketing. Possible targets may be:
Search Engine Marketing is not usually a separate marketing discipline, but rather is embedded in online marketing concepts. Therefore, parts of the SEO can be important for content marketing, storytelling or newsletter marketing. In content marketing, for example, it is important to syndicate content. However, if important SEO rules such as the search for suitable keywords or a meaningful headline structure for individual campaigns are not observed, there is no chance that these texts or content elements will not be found so well. At the same time, SEA measures can also strengthen other online marketing measures. For example, it is conceivable that Google AdWords could be used to promote a new website on the Internet until the SEO measures take effect.
Search Engine Marketing can be part of an entire marketing concept. Like there, a concept for SEM should also include important preliminary considerations such as SWOT or PEST analysis. The big advantage of SEM is that the monitoring of online activities is much easier than with conventional marketing measures via print or TV. Among other things, the scope of SEA activities can be precisely tracked for customers and service providers using web analytics.
Without search engine marketing, companies today usually have little chance of being successful on the web. Due to the increasing importance of the Internet and the increasing technical and professional requirements, there are more and more aspects that have to be considered. Whether companies rely on external service providers such as SEM agencies or provide in-house know-how is irrelevant. Basically, however, the following applies: If you have the current and well-founded know-how in search engine marketing, you can always gain an advantage over your competitors.