SEO = search engine optimization - Tricks to increase your search engine rankings positions SERPs SEM - search engine marketing via sponsored search results, PPC, CPM etc.
Meethere and sipltech are both correct. And just want to add that SEO is still considered part of SEM. SEM tackles everything, paid and non-paid. SEO is concentrated on organic results.
SEO - practices to ensure that your website appears in the top ranking on SEs. SEM - the process of obtaining visitors through SEO
SEO - Optimizing a website or webpage to achieve high rankings in the organic results. SEM - An umbrella term for all aspects of marketing through search engines. Both SEO (Organic) and PPC (Sponsored).
both seo and sem main aim is to increase quality traffic, generate sales resulting in more revenue for the site. SEO focusses on raising the site's position in search engines original search results through the use of some keywords. SEM focusses on getting immediate hits to the site through results coming in sponsored search on using some keywords. (PPC)
SEO is just a part of SEM, because in SEM, you must consider not just the position of your site's SERP, but also the uniques and returning traffic from them. You must not just consider getting traffic from content, but also from your web design.
Personally, I don't consider SEO to be a part of SEM. The goal for SEO is to be ranked high on the search engines. SEM, like previously mentioned, would include PPC, Paid Inclusion, etc. Bigger companies/sites need to include SEM in their plans, whereas smaller businesses can get away with just SEO
Search Engine Optimization (SEO) is the process of improving the volume and quality of traffic to a web site from search engines. Usually, the earlier a site is presented in the search results, or the higher it “ranks,†the more searchers will visit that site. The leading search engines, Google, Yahoo! and Microsoft, use crawlers to find pages for their algorithmic search results. Search Engine Marketing (SEM) is a form of Internet marketing that seeks to promote websites by increasing their visibility in search engine result pages (SERPs). According to the Search Engine Marketing Professionals Organization, SEM methods include: search engine optimization (or SEO), paid placement, and paid inclusion. To know more in detail : SEO vs SEM An article on the differences between search engine marketing and search engine optimization.
SEO is free SEM is paid if we want to hire seo then we pay to him for his/her work, when we also pay to Google and other search engine it's called sem( we can also hire a seo for sem).WE also can take get PPC service in yahoo and other search engines.