What's the difference between creating a 'Content' or a new 'Pages>>Add Custom Page'? I'm working on a pre-established site. I've noticed that I can add new 'Content' (Content>>Add Content) or I can add a new Page (Structure>>Pages>>Add custom page).

The site seems to use an indiscriminate amount of both, though Content seems to be used more with templates for blog and product pages.

What are the differences between the two and in what scenarios should I use them?
Structure -> Pages menu link comes from ctools and it allows you to create more complex pages than your basic drupal core ones (Content -> Add Content link). This may include components comming from other modules (Panels, Fieldable Panel Panes, Views Content Panes and many other that intergrate with Ctools).

With Page Manager (and panels in general) you can create more complex versions of a page (or more, if you use the right contexts).