1. content: information (text,images, etc)
  2. configuration: 基本不会再变的站点基本信息
  3. state: Information of a temporary nature about the current state of your site, such as the time when cron jobs were last run.
  4. session: Information about individual site visitors' interactions with the site, such as whether they are logged in and their cookies. This is technically a subtype of State information, since it is also temporary.