Let’s try and break it down simply. SEM or Search Engine Marketing is the broader sense of marketing your business online. It is more of a universal phrase, it can include search engine optimization (SEO, which is organic in it’s existence) in addition to or apart of a marketing strategy that also calls for Pay Per Click (PPC)/Paid Search Advertising which is just that, purchased advertising. Those are the two primary avenues of SEM, and the other components related to online marketing are a way to bolster those fundamental marketing efforts, those like websites, landing pages, and integrated advertising and branded social media tie-ins.
The simple answer is SEO is one way to market your business online, if done properly it raises your websites visibility allowing it to be found nearer to the top search engine results pages, thereby giving your business the upper hand.
Sounds easy enough, right? Actually it is a complicated process, one that relies on a lot of things working together, we talk about this more here. Outcomes are typically very favorable, but it has to be stressed that this is a truly organic, ever evolving process, meaning you can not buy or pay to influence your search ranking. They say rome wasn’t built in a day, and there are no quick fix actions you can do to change your ranking overnight, it takes time. If this has been promised to you, please check out this great source here, this one here, or research others you trust. That said, if a thoroughly planned and rigorously executed search engine optimization strategy is employed, you will see forward movement!
Search rankings are controlled through complex (and tightly guarded) search engine algorithms. These are user influenced, with the intentions of anticipating human search terms and meanings, and are designed to deliver the most relevant, quality content through any given SERPs (search engine results pages), while simultaneously weeding out (and subsequently penalizing) those websites that attempt to use unethical practices aimed at manipulating search results.
The honest answer here is no, search engine submission is not necessary. This is due to the nature of how most search engines, including Google, crawl and index pages by following links. Even if your website has one inbound link from any other previously indexed page, this can be enough to identify and index your page. Once that happens, if your newly indexed page links to other pages on your website the search engine will crawl those links as well and voila!
The rest of the story is that you can submit your website, and this is typically a free service the major search engines offer. However, it is important to note this may not effect if or when your site gets crawled. It is also not something that needs to be done any more than once.