diff --git a/apiclient/discovery.py b/apiclient/discovery.py
index 328d970..954ad6c 100644
--- a/apiclient/discovery.py
+++ b/apiclient/discovery.py
@@ -45,13 +45,21 @@
     import json as simplejson
 
 
-class HttpError(Exception):
+class Error(Exception):
+  """Base error for this module."""
   pass
 
 
-class UnknownLinkType(Exception):
+class HttpError(Error):
+  """HTTP data was invalid or unexpected."""
   pass
 
+
+class UnknownLinkType(Error):
+  """Link type unknown or unexpected."""
+  pass
+
+
 DISCOVERY_URI = ('http://www.googleapis.com/discovery/0.1/describe'
   '{?api,apiVersion}')