SEO may generate an adequate return on investment. However, search engines are not paid for organic search traffic, their algorithms change, and there are no guarantees of continued referrals. Due to this lack of guarantees and certainty, a business that relies heavily on search engine traffic can suffer major losses if the search engines stop sending visitors. Search engines can change their algorithms, impacting a website's placement, possibly resulting in a serious loss of traffic. According to Google's CEO, Eric Schmidt, in 2010, Google made over 500 algorithm changes – almost 1.5 per day. It is considered a wise business practice for website operators to liberate themselves from dependence on search engine traffic. In addition to accessibility in terms of web crawlers (addressed above), user web accessibility has become increasingly important for SEO.
So if you think about it, SEO is really just a process of proving to search engines that you are the best site, the most authoritative, the most trusted, the most unique and interesting site that they can offer to their customer - the searcher. Get people to talk about you, produce good quality content, get people to link to you, and Google will be more confident that you are the best result that they can offer to their searchers, and that’s when you will start ranking on the first page of Google.
The key distinction is that SEM is a broader strategy which combines paid search and SEO tactics. So, if you set up a PPC advertisement, but don't take steps to optimize the ad or the website it links to, this is a standard example of paid search. However, if you optimize your site and your search content with solid keywords, then you put money behind it to boost it on targeted audience's search pages, then you're dabbling in SEM.