A whole mega-industry has been built up around Search Engine Optimization or SEO so the term gets used, and over used, quite a bit.
The basic definition is that it is a set of techniques that help optimize your content so that it ranks higher in natural organic search results.
It is not however and should not ever be used, to trick or game search engines into ranking your site higher than it deserves to be.
Just remember, the Google giveth and the Google can taketh away!
So play nice with Google or suffer the consequences.
On Page SEO
On Page SEO simply means doing everything possible to help Google and other search engines know what each page of your website is about. This way, they will index the keywords that best describe your pages and establish the relevancy of your content accordingly.
Here’s a quick example. Putting the title of a blog post in an H1 html header indicates to the search engine that this is the title of the article and that the key words contained in it have the most relevance to the page. Put the same title in bold and Google will not understand that it is the actual title of the article.
Another good example is how to properly use pictures on your website. A picture may be better than a thousand words, but Google can’t read and index pictures. Therefore, it’s important to ensure that you include text in the background that describes the picture so that Google can understand what it is a picture of and index the information accordingly.
Remember your site needs to be awesome. This is not the same as looking awesome.
If your site is loaded up with wicked cool looking graphics and lots of stunning video and flash, but you don’t take the time to optimize your content, then to Google it will look like a blank page. And blank pages aren’t relevant.