What is content document in salesforce?

Hi,

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.

Supported Calls
delete(), describeLayout()describeSObjects(), query(), retrieve(), search(), undelete(), update()

you may refer tohttps://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_contentdocument.htm