What is appbase and docbase

What is appbase and docbase ?
Ans :
appBase :
The Application Base directory for this virtual host. This is the path-name of a directory that may contain web applications to be deployed on this virtual host. You may specify an absolute path name for this directory, or a path name that is relative to the $CATALINA_BASE directory. See Automatic Application Deployment for more information on automatic recognition and deployment of web applications to be deployed automatically.If not specified, the default of webapps will be used.
docBase :
The Document Base (also known as the Context Root) directory for this web application,
or the path name to the web application archive file (if  this web application is being executed directly from the WAR file).
You  may specify an absolute pathname for this directory or WAR file, or a  pathname that is relative to the appBase directory of the owning Host.
If a symbolic link is used for docBase then changes to the symbolic link will only be effective after a Tomcat restart or by undeploying and redeploying the context. A context reload is not sufficient.
Do not choose a docBase that starts with your Host’s appBase string. The default appBase is “webapps” so do not choose a docBase like “webapps-foo.” Doing so will lead to deployment errors: see Bugzilla for details.
The value of this field must not be set when the Context is configured using a META-INF/context.xml file as it will be inferred by the automatic deployment process.