The main reason for using search engine optimisation (SEO) is to improve your position on rankings pages like Google, yahoo etc. All these search engine programs have different methods of ranking pages and are constantly changing to improve the quality of their results. Getting high rankings (top 10) on these sites can massively improve your sites' traffic, but if your site is badly designed you may find that not many are listing you in their top rankings. Competition is fierce on attaining these rankings and may take months before you can even achieve 1st page rankings. Through constant monitoring, submission and changes on your site you may make the top 10.
Be wary of companies that offer instant results.