Seo - (search engine optimization), the process of simply letting other web surfers know that you're there and are open for business.This is achieved by helping the search engines (Google, Yahoo, Ask, Bing and several other less known) understand what your site is all about. The search engines obtain information from your pages using a special program called a spider or robot. The info is provided with the use of special meta tags, headings and the written content of your site.
So for example, a site providing secretarial services would have it's contents read over and stored by the search engines for future use. Then when someone searches for secretary, your results would be retrieved and displayed on a search results page. Simply really.
A good link will genuinely be of interest to a site visitor and in some way connected to the theme of your own website. It shows relationship with others on the net and if your website's content is interesting and up to date then others may link to your site, increasing the search engine's awareness of your site and hopefully pushing your ranking up.
SEO isn't just about tags and headers it's also about content and how you convey the information to the site visitor. Well written content will provide the search engines with a clearer understanding of what your site is about, how it relates to similar subjects and how best to categorize the site.
How much will it cost? - Additional SEO work - prices depend on your requirements
Some web developes or SEO businesses will charge for the following: