There are a variety of search engine optimization services which offer solutions for a variety of ranking issues, and deficiencies. Depending on your goals, and needs one, or a combination, of the below services may be right for your website.
A search engine optimization audit can come in a varying levels of detail and complexity. A simple website audit can be as short as a few pages long, and would address glaring on-page issues such as missing titles, and lack of content. On the other hand other end of the spectrum, a comprehensive website SEO audit will be comprised of dozens of pages (for most larger sites it will be over one hundred pages) and address even the tiniest of website elements which may potentially be detrimental to the ranking-ability of a website.
On-page or on-site search engine optimization refers to SEO techniques which are designed to implement the problems and potential issues that an SEO audit uncovers. This is something which should always be part of all good SEO packages. On-page SEO addresses a variety of fundamental elements (as they relate to SEO) such as page titles, headings, content and content organization, and internal link structure.
As with a website SEO audit, there are basic, as well as comprehensive services when it comes to on-page search engine optimization. At the most basic level, an on-page optimization campaign can be a one-time project which includes recommendations developed through an audit, and the implementation thereof. This type of on-page optimization would generally target the home page and a few other important pages on the website. More comprehensive on-page search engine optimization campaigns will use the findings of a highly detailed website SEO audit, and monitor results to guide ongoing changes to the on-page optimization.
Link development is one of the most controversial and often talked (written) about topics of the search engine optimization industry. Since backlinks are the most vital component of any search engine optimization campaign, and at the same time the most time consuming and consequently most expensive (assuming they are good quality links and not just random directory submissions and blog comment spam) part, inevitably, there are many service providers who offer inexpensive link building services in order to attract and impress potential clients. Such schemes include large volumes of directory submissions (e.g., 200 directory submissions per month), worthless blog and forum comment spam (e.g., 100 blog links per month), or article writing and submissions which result in extremely poor quality content published on equally low-quality article directories which contribute in no positive way to ranking improvements. So if someone is quoting you a $500 per month search engine optimization services which includes large volumes of directory submissions, blog posts, articles, blog/forum comments and so on, all you will be doing is throwing your money away. This is not to say that you can't get link-work for $500 per month; however, it won't be for a large volume of links.
Good quality link development work focuses on quality rather than quantity. A well researched and relevant, good quality link is worth many times more than hundreds of free directory submissions.
The fundamentals of link building are, have always been and always will be, based on good quality (i.e., useful, interesting, entertaining, educational) content. Because if there is no good content on your site that people can link to, it will be very difficult to convince them to do so.
SEO content writing is somewhat of a misnomer--it really should be replaced with high quality and well researched content writing. The term "SEO content writing" implies that there is a secret writing formula which turns plain everyday text into something magical that gets the attention of the search engines--this could not be further from the truth.
If you are looking for content writing services which will help your website attain higher rankings, what you are really looking for is high quality and well written content, and not SEO content. SEO content is what you would get from a writing sweatshop or someone who cannot afford to write good content because they are only charging you $12 per "article".
SEO content writing as a service can be useful, if shortcuts are not taken, and the content is not expected to perform magic. Well written, interesting and useful content will inevitably be found, and get attention on its own merits; however, it also helps lay the foundation for a successful link development campaign.
Code optimization is a service you can expect at the highest levels of search engine optimization services, as it involves an overhaul of your website HTML. The optimization of your HTML can impact search engine rankings in two ways. First, it can help alleviate code-clutter, and present your content in an easy-to-understand (for machines, that is, search engine algorithms) format. Second, it can help reduce the load-time of your website pages, so that search engine spiders don't have to wait around while your page loads (because it's too long, or has too many images, etc).
A comprehensive search engine optimization campaign will have all of the above elements, but it will also incorporate other important services such as keyword research, ranking reports, traffic reports, and conversion tracking.