Internet Social Media and PR has basically created an entirely new method for marketing websites. Google the search giant of the internet has undoubtedly cornered the market for mainstream search. It is the "one percenters" of the internet that have basically figured out how to utilize search engines like Google in order to raise awareness and drive targeted traffic to websites.
Knowing what keywords the new webpage is going to be focusing on is a primary concern. Another "need to know" is, what are the keywords being targeted in the content that is linking to and from our new webpage. How to target keywords is an entire subject by itself, this will be discussed at a later date. But, for now a good rule of thumb is, use your keywords early and often. What this means is, don't crowd every sentence and line with a keyword, instead use the words "naturally" in your titles, opening sentences to paragraphs, menus/list items. So basically anywhere that you can "naturally" use a keyword, without spamming your own website, is a good place.
What does all this have to do with Social Media? Well, through the use of all the different forms of social media a person can utilize the power of others to generate links to their webpages. All of the incoming links from webpages outside of a website are tracked by the search engines. Most advanced search engines such as Google, Yahoo, MSN Ask.com, etc., track the linking structure of the internet. Now if an individual webpage is optimized for a set of target keywords, and the incoming/outgoing links to other webpages have content with similar keywords, the search engines will then give that webpage a higher ranking for those certain keywords. So you have to think about the target keyword set, and the keywords used in the incoming/outgoing links and their keywords in order to begin understanding how Search Engine Marketing works.