diff --git a/api-guide/source/general_info.rst b/api-guide/source/general_info.rst
index 2180e96715..9745c29304 100644
--- a/api-guide/source/general_info.rst
+++ b/api-guide/source/general_info.rst
@@ -86,6 +86,17 @@ several key concepts:
    reason. Such a separation is possible in Nova if an admin carefully sets up
    availability zones for that, but it is not the default.
 
+-  **User data**
+   A user data file is a special key in the metadata service that holds a file
+   that cloud-aware applications in the server can access.
+
+   Nova has two ways to send user data to the deploying instance, one is by
+   metadata service to let server able to connect to its metadata through
+   a predefined ip address (169.254.169.254), then other is to use config
+   drive which will wrap metadata into a iso9660 or vfat format disk so that
+   the new deployed server can consume it by active engines such as cloud-init
+   during its boot process.
+
 Networking Concepts
 -------------------