Spot on MalaysiaDN.
The Offsite optimization is slightly more complicated than just linking to your site though. It's important to carefully select your Anchor Text (vary this while using the same keywords). Also, ensure that the sites you link from have a good level of relevance, good content, and a low number of outbound links. Also, linking from lots of pages on the same site will not contribute as much as you think. Although useful for crawling, SE's will usually only add distinct IP's to the count. One well placed link is all it takes. So if you purchase links, it's best not to pay more for a "run of site" text link.
Keyboard Cowboy simplified this somewhat nicely, but when going the whole hog, there are lots of things that Google will take into account. This includes the page title of the linking site, the content surrounding your link, the url of the site linking to you, and much more. Some of these things are out of your control unless you have a very influential site. Even then, trying to fulfill all these is not a nice task. Ultimately, it's not worth it for the average site, but for very competitive keywords, you have to execute this with care.
You can then get into deep linking, applying & structuring subdomains , IP content distribution (very powerful when used with subdomains and will require dedicated and/or multiple servers), but this very much depends on the application and the scale of your project.
All of this should be considered a campaign, and be well planned. If you are intending to do this properly, and comprehensively, it can take a great deal of time and work. Note that this isn't necessary for all sites.
If you wish to learn abit of SEO, it's a good idea to take one of your sites, and implement and test each of these factors one by one. You can then see what works, how it works, and why it works.
Calvin