Let's say you want to make a change to the QuickLaunch menu for a single page on your SharePoint site. This sounds complex right? Well, it is as complex as you want to make the job.
However, for one-off changes to the content of a SharePoint page you can add a "Content Editor WebPart". You can find the "Content Editor" in the webparts screen. The Content Editor lets you add HTML into a page.
Site Actions --> Edit Page --> Insert --> Web Part