[development] Metadata question

Hello, I am trying to figure out how metadata is used in Drupal and I'm a bit lost. I am going to add new metadata to objects (nodes?) for a project I'm working on, and I came across flexinodes, node, taxonomy, relationships, and I need someone to clarify things up some for me. Are all things stored as the same object? Is an attachment (pdf, doc, jpeg, etc) stored as the same thing? Does it have metadata? Does regular content (non-attachment) also have metadata? Is it also stored like attachment metadata (if it has some)? Thank you, Gabriel