tactics
An SEO/SEM marketing company operates under the assumption that their services actually help your site increase in Page Rank. They can never provide concrete or quantifiable results that their services have actually done anything, because this is impossible, but they will never tell you this. The Google Page Rank Algorithm, the piece of code that determines how your site ranks against every other site on the Internet is a proprietary piece of Google software, which means that Google will NEVER tell you or anyone else how it works. All the information used to design a site SEO/SEM purposes is pure speculation.An SEO/SEM marketing company will contact you telling you how your site could be #1 in searches and they speak directly to your desire to be the best out there. They'll provide you with analysis and remediation techniques in a report that you probably have to pay for. This report takes them time to generate, meanwhile your site is active on the Internet and people can visit it, including the Googlebot.
The Googlebot is the "robot" that spends all day every day scanning every single individual site on the Internet. It reads through every aspect of your page and records the vitals about the page, including its Topic, its Title, its Keywords, how frequently those Keywords are used within the content, links to your site (backlinks) and links from your site to somewhere else. With this information (and a bunch more that we'll never know about), the Google Page Rank Algorithm calculates a specific Page Rank for your site. The higher your Page Rank, the higher your site will rank in queries. The Googlebot will continue to poll your site about 1-2 times per month, forever.
SEO/SEM companies can configure your site to feed good information to the Googlebot, but they can only go so far before the Googlebot classifies your site as using scammer tactics. These include stuffing your content full of keywords, hiding links, placing content with the same color as its background (to hide it), any many more.
As the SEO/SEM company begins its procedures of re-writing your page and stuffing it full of SEO/SEM goodness, people continue to visit your site and some of them might even place a link on their site to your site (a backlink). Each backlink adds to your Page Rank and increases your site's visibility, although it may seem like this was from the services of the SEO/SEM company, and they will claim it as their own doing if you cant prove otherwise.