diff --git a/doc/source/index.rst b/doc/source/index.rst
index 8c5fe3f52..139c21cec 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -128,12 +128,27 @@ API reference
 Modules reference
 =================
 
+Zaqar is composed of two layers:
+
 .. toctree::
    :maxdepth: 1
 
    transport
    storage
 
+The **transport drivers** are responsible for interacting with Zaqar clients.
+Every query made by clients is processed by the transport layer,
+which is in charge of passing this information to the backend and then
+returning the response in a format understandable by the client.
+
+The **storage drivers** are responsible for interacting with the storage backends
+and, that way, store or retrieve the data coming from the transport layer.
+
+In order to keep these layers decoupled, we have established that
+**checks should be performed in the appropriate layer**. In other words,
+transport drivers must guarantee that the incoming data is well-formed
+and storage drivers must enforce their data model stays consistent.
+
 Indices and tables
 ------------------