Standard Objects are best described as components of Documents. They are blocks of text with associated headings and image that are defined once and can be used again and again across many pages.

An example might be a 'special offer of the month' that you want to appear on a number of pages. Each month you would just change the Standard Object and it would update all the other pages.

All Documents (which can incude web pages,intranet pages, jobs, emails, and so on) can have Objects attached to them. If an attached Object is a Standard Object, then it is stored centrally: one Standard Object can update many pages.