What are the difference between attachments and content documents in Salesforce?

Content Document Represents a document that has been uploaded to a library in Salesforce CRM Content or Salesforce Files.This object is available in API versions 17.0 and later for Salesforce CRM Content. This object is available in API version 21.0 and later for Salesforce Files.

Files attached to emails and events can be saved in Salesforce as Content documents (Files) or as Attachment objects, the former being the default way. In either case the files are stored in Salesforce under Notes & Attachments of the primary associated object(s)