An overview is an assessment of a site and an indication of what checkpoints are passes or fails. This is a useful service if you want to benchmark how accessible your site is against our See it Right or Surf Right or against WCAG 1.0 or 2.0 and see how you are doing.
The report that follows an overview, shows you which accessibility issues your site passes or fails, but it doesn't provide in-depth analysis or recommendations or follow-up recheck.