Site Optimisation is the process of improving the volume and quality of a website and its traffic.
Some Site Optimisation efforts may involve optimising a site's coding, presentation, and structure, without making very noticeable changes to human visitors, such as incorporating a clear hierarchical structure to a site, and avoiding or fixing problems that might keep search engine indexing programs from fully spidering a site.
Other, more noticeable efforts, involve including unique content on pages that can be easily indexed and extracted from those pages by search engines while also appealing to human visitors.

Search Engine Optimisation (SEO) is marketing by understanding how search algorithms work and what human visitors might search for, to help match those visitors with sites offering what they are interested in finding.

Search Engine Marketing (SEM) is most important for the businesses who sell goods and services online or who use their websites to generate sales leads. Other goals for SEM including: building a brand, enhancing reputation with investors, generating media coverage, driving traffic to physical business locations, and to promote their ideas.

Pay Per Click is an advertising technique used on websites, advertising networks, and search engines.
Advertisers bid on "keywords" that they believe their target market would type in the search bar when they are looking for their type of product or service. These ads are called "sponsored links" or "sponsored ads" and appear next to and sometimes above the natural or organic results on the page.
The advertiser pays only when the user clicks on the ad.