To understand search engine optimisation and start increasing your search engine rankings you must first understand the two fundamental parts of optimisation which are:
On-page optimisation is the optimisation of factors that are fully controlled by you inside of your website and each webpage. As the owner you have full access to your website and you can add/remove content or pages, change titles, link internally and externally and much more as you please. On-page optimisation is very important because it informs search engines (such as Google, Bing & Yahoo) what your website is about, for which they will rank you on. If search engines cannot clearly understand what your business is about or reads your web pages are not relevant enough then there is a good chance you will be rankly poorly, if at all.
An Off-page factor is everything else that is off the page or off site. In a nut shell this refers to links. Links from other websites pointing to your website count as a vote for your website. Each link you get acts as another vote in the Search Engines eyes. To increase your rankings it is absolutely essential to get as many links as possible from trusted sites.
Whenever you're trying to find more links for your website you must consider the quality of each link as not all links are equal and some links from low quality websites may have minor or no influence on your rankings at all. The most effective kinds of links you can obtain are found from trusted websites such as colleges, universities and newspapers. It is even possible to gain quality links from some directories such as dmoz and Yahoo.