SEM which stands for Search Engine Marketing is a combination of organic and paid results, while Search Engine Optimization is only results that are gained by natural means and are free of cost. SEM is not free. If you want to improve the sales of your business then you need to include SEM in your strategy. Results obtained from SEM are obtained very fast as compare to the results obtained by SEO. SEO is a part of SEM technique. SEO is all about making a website appear in natural or organic results provided by search engines. Almost 90 % of the traffic for a website is gained by Search Engine Optimization of that website.
A PPC campaign that stands for Pay-Per-Click campaign is the main method of Search Engine Marketing. In PPC campaign an advertiser needs to pay money for each click. Advertisers bid for the keywords and key phrases that are most likely to bring more traffic towards their site according to them. When users search for the keyword that is relevant to the ads of the advertisers, then their ads are displayed in the search engines. Generally the advertisers, who pay more money than other advertisers, get higher ranking in search engines.
Pay-For-Inclusion abbreviated as PFI, is a method of getting a guarantee of being listed in search engines which requires a webmaster to pay a fee for that. Just getting listed is not a guarantee of earning profit through PFI. If you want to earn money through PFI, then you need to optimize your website for search engines.