Search Engine Optimization or SEO is the art and science of getting your website to rank high in search engine results for keywords that are pertinent to your particular niche.
SEO has become highly competitive in recent years as the number of sites on the internet has increased exponentially. Nevertheless, it is still possible to rank for many keywords if you exert some effort.
There are really three primary categories that effect your SERP rankings, namely, on-page factors, off-page factors, and social media activity.
On page factors will include things like the manner in which your site is constructed, and the type of content that is on your site. Generally, larger sites do better than smaller ones and sites with original, compelling content will do far better than sites with duplicate or spun content.
Content that is original,interesting, and has images and video in it is crucial to ranking well in the SERPs. If you don't have this type of content, you will not receive favorable rankings from Google.
Other on-page factors will be things like the load time of your site and the content in your title and meta tags.
Off-page factors generally consist of the types of links that you have to your site. Links have long been a very large ranking factor, and they continue to be a factor in organic search result rankings. However, Google closely scrutinizes the types of links that are pointing to your site. Google only wants to see "natural" links to your site. They really frown upon people joining blog networks and other means of artificially getting links to their sites. In fact, if it appears to Google like you have unnatural links to your site, they will not give you high rankings in their search results and they may go so far as to de-index your site.
Of course links from sites that are high PR and considered authority sites are very valuable links. Links from sites that are low PR are not so valuable. Many links that are easy to obtain, such as profile links have been devalued considerably by Google in recent times.
In addition, the anchor test used in your links has to have considerable variation. If it appears to Google like you are optimizing a particular page in your site to rank for a keyword, they may slap you with an over optimization penalty.
Social media activity is also used as a factor in determining your rankings in the SERPs. If there is a large social media buzz with regards to the content on your site, your rankings will improve significantly. Social media sites like Facebook, Twitter, Google Plus, Pinterest, and LinkedIn figure prominently in search engine results.
Google has also created a mechanism for site owners to author their content in Google Plus. This feature makes it difficult for someone else to copy your content and claim it as their own. It can also help you to increase your ranking in Google's search engine results.