It is the process of making the content of your website more accessible to the web crawling, data collecting applications (spiders/web-bots/robots) that collect the information that Google needs to index the content of your web pages.
Google bots crawl your web site and record many factors about your pages including the page title, the page description (a brief summary of the page content that appears in the Google results) and a scan of keywords and phrases that occur within the page itself. It will also record links to external sites (the logic being that the more incoming links to your pages - the more importance is given to the content) and hence the higher the page rank.
Some (less scrupulous) SEO companies use "underhand" methods to fool these Google bots into exaggerating the page relevance of your content to generate more incoming traffic to your web site. These can include setting up hundreds of spoof web sites with links to your site to boost the perceived "page rank" or hijacking legitimate sites (including your own) with malicious software to generate more spoof links, over use of keywords and phrases, hidden text (very small text the same colour as the background) - invisible in the browser but readable by the web bots.
Wrong, there is no point in having people visit your site if 5 seconds later, they use the browser back button to leave it again because the content is neither relevant to the search query or not presented in a user friendly format.
Also, when discovered (and it will be), can lead to your site being removed from the Google index completely, which will damage both your business and your reputation.
All of the "Black hat" SEO companies will ask for some kind of payment up front or ask you for your credit card details, this should trigger the warning signs in your head.
Just try and get your money back when you fail to get the results they promise or worse still, your site gets pulled out of the Google listings!!
As I build your web site, each page is coded following Google's Webmaster Guidelines and W3C web standards to ensure that your web pages contain user friendly, content rich and engaging information. Promoting your site through quality related sites will ensure your site gains quality incoming links from trusted sources naturally.