There are two types of SEO – on-site and off-site.
On-site SEO relates to things that you do… on your site. These might be technical changes, content changes, structural things, etc. These are the things that are the easiest to control and to influence, because you control your site.
Off-site SEO relates to things that are happening on other people’s websites. This includes link building, social networking, interfacing with third party channels. These things are harder to work on, because you won’t have as much control, but they can often be more beneficial.
Think of it this way…
You’ve built the best shop in the world (on-site SEO) but nobody bothered to put an advert in the paper or tell anyone you were open (off-site SEO). Your shop is not very busy…
It’s easy to fall into the trap of only doing one kind of SEO, usually the one that is easiest for you or the one where you get the quickest results with the least effort. But, by doing this, you’re limiting the number of opportunities you have to succeed.
My advice is this – do both on-site and off-site SEO.
Typically, on-site SEO will pay back quicker – especially on a website that has structural, technical, or content issues. Off-site SEO may take longer to pay back, will pay back bigger, generally speaking.