For those of you new to promoting websites on the internet, SEO stands for search engine optimization. SEO is the act of optimizing a website so that the search engines know where to index or place the site correctly in their search results pages.
The job of a search engine like Google, Yahoo, or Bing is to serve up the most relevant results to any given search. In order to do that it crawls and reads the pages of every website that it's aware of on the Internet, and indexes it based on what it sees. Proper SEO helps search engine's do their job.
There are several aspects to good SEO. There are on-site and off-site factors that must be taken into consideration. On-site factors are things such as the title of the site (URL), page titles, internal links (links within the site itself), use of H1, H2, H3 tags, the bolding of important words, Alt tags on image, etc ..
It's a great idea to incorporate all of your on-site factors when the website is being built. That way you'll have a good base to build on. A good website designer will include all the things that the search engines are looking for, as the site is designed and developed, but it is also possible to take an existing site and add any SEO factors that are missing.
Off-site factors include things such as links to your site from other sites (the higher the authority the better), anchor tags from other sites (the words that link to your site from other sites), links from video sites, topical relationship of sites that link to yours, etc. Off-site factors are also links to your site or blog from other blogs or comments on other blogs.
Off-site factors can come from other site owners linking to your site because they think the content would be of interest to their readers. It can also be self-generated by creating your own off site content like extra off-site blogs, Hub pages, Squidoo lenses , and EzineArticles like this one.
On-site factors used to have the most important factors on a website. That is quickly becoming old-school because there are more and more websites competitive for the first pages of the search engines. On-site factors are still very important to getting indexed correctly; especially for local or low competition keywords. But now it seems they remain purely an important core of a good website.
With competition for the first 10 spots on Google getting stiffer and stiffer off-site factors have become more and more important. As a matter of fact it's impossible to compete in today's world at a national or global level without making sure that all of your off-site factors are being updated and added to on a regular basis.
To sum things up: For low competition keywords, especially where people are typing a location into their search, like "Chiropractor in Lake Worth Florida," on-site SEO may be all you need. But for national and global searches, or highly competitive search terms (which are the majority of sites on the web today) it's going to take more work and plenty of off-site SEO.