Search Engine Optimisation is the process of making improvements on and off your website to gain more exposure in search engine results.
It can be divided into two main areas:
1. On-page optimisation, which covers what can be done on the pages of the website itself.
2. Off-page optimisation, which covers activity that takes place outside of the boundaries of the website. The most important are: (a) Link Building, (b) Social Media and (c) Social bookmarking.
In order to understand what changes are likely to help it useful to consider what it is that search engines are trying to do. They are trying to find and understand all the content on the web and deliver relevant and authoritative results based on any phrase a user may be searching for.
Relevance is the similarity between the phrase entered by the user and the content on the page. The closer it is the better. Whilst search engines undestand semantics and themantics direct matches will be deemed the most relevant. This is a very important concept and gives rise to long tail keyword opportunities.
Authority is the credibility that Google places on the content on the page. A key contributing factor to this is the number of links to the domain and the page from outside websites and the quality of those websites.