diff --git a/gallery_dl/extractor/message.py b/gallery_dl/extractor/message.py
index 04aedfc4..1831620a 100644
--- a/gallery_dl/extractor/message.py
+++ b/gallery_dl/extractor/message.py
@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015-2017 Mike Fährmann
+# Copyright 2015-2018 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -8,6 +8,42 @@
 
 
 class Message():
+    """Enum for message identifiers
+
+    Extractors yield their results as message-tuples, where the first element
+    is one of the following identifiers. This message-identifier determines
+    the type and meaning of the other elements in such a tuple.
+
+    - Message.Version:
+      - Message protocol version (currently always '1')
+      - 2nd element specifies the version of all following messages as integer
+
+    - Message.Directory:
+      - Sets the target directory for all following images
+      - 2nd element is a dictionary containing general metadata
+
+    - Message.Url:
+      - Image URL and its metadata
+      - 2nd element is the URL as a string
+      - 3rd element is a dictionary with image-specific metadata
+
+    - Message.Headers:  # obsolete
+      - HTTP headers to use while downloading
+      - 2nd element is a dictionary with header-name and -value pairs
+
+    - Message.Cookies:  # obsolete
+      - Cookies to use while downloading
+      - 2nd element is a dictionary with cookie-name and -value pairs
+
+    - Message.Queue:
+      - (External) URL that should be handled by another extractor
+      - 2nd element is the (external) URL as a string
+      - 3rd element is a dictionary containing URL-specific metadata
+
+    - Message.Urllist:
+      - Same as Message.Url, but its 2nd element is a list of multiple URLs
+      - The additional URLs serve as a fallback if the primary one fails
+    """
 
     Version = 1
     Directory = 2