In Internet marketing, search engine marketing, or SEM, is a set of marketing methods to increase the visibility of a website in search engine results pages (SERPs). The three main methods are:

• Search engine optimization (SEO), or improving rankings for relevant keywords in search results by rectifying the website structure, and content such that they could be easily read and understood by the search engine's software programs. It is seen that website containing the latest trends and updates are first available to the visitor.
• Search engine advertising or paying the search engine company for a guaranteed high ranking or an ad displayed aside the results (commonly known as pay per click advertising).
• Paid inclusion, or paying the search engine company for a guarantee that the website is included in their natural search index.
Search engine marketers are experts and firms who explore of weaknesses and strengths in the methods and individual products to find the best way to promote a particular website in search engines.
Search engine optimization (SEO)
SEO is a process of preparing your website and its constituent pages for prime positioning on search engines, using specific keywords. SEO is actually very complex, as it changes on a regular basis.
Search engine optimization is usually divided into two categories of tasks: on-site optimization, otherwise known as natural optimization and off-site optimization or link building. On-site optimization focuses on improve your website’s layout, content, etc. Off-site optimization improves your search engine position by building up links pointing to your website from other websites.
Search engine advertising

Advertising with search engines could be further classified as Advertising based on a keyword search and Advertising based on content context.

Advertising based on a keyword search:

Advertising based on a keyword search could take place through a search engine such as google.com, or a search engine partner site, such as shopping.com. For example, Google offers a service called AdWords, which allows companies, for a small fee, to have a link to their website featured when a user searches a specific keyword which the company specified.

Advertising based on content context:

Many search engines (e.g. Google, Yahoo! Search) have partner websites with specific content. The websites agree to let the search engines place content-specific advertising on their website, in return for a fee. The search engine then finds companies interested in advertising on websites with their desired content. For example, an online dog food retailer might have their advertisement placed on a site about dogs.
Both of these advertising formats allow advertisers to target specific users with certain interests. Generally these advertisements are paid for based on either a pay per click campaign or an impression based campaign. Pay Per Click listings work because they put your company in front of people looking for your products or services.

When a potential customer arrives at your web site, they have already searched for products or services you offer. This makes them a pre-qualified lead, with a high chance of them purchasing your products or services.

Paid inclusion

Paid inclusion is a search engine marketing product where the search engine company charges fees related to inclusion of websites in their search index. Paid inclusion products are provided by most search engine companies, the most notable exception being Google.

This is different from pay per click advertising because the inclusion is guaranteed but not placement.