diff --git a/LetoDMS_Core/inc.ClassDMS.php b/LetoDMS_Core/inc.ClassDMS.php
index b025a31c8..cfa9b4d1d 100644
--- a/LetoDMS_Core/inc.ClassDMS.php
+++ b/LetoDMS_Core/inc.ClassDMS.php
@@ -33,8 +33,9 @@ require_once("inc.ClassNotification.php");
  * objects representing the entities in the dms, like folders, documents,
  * users, or groups.
  *
- * Each dms has its own database for meta data and data store for document
- * content. All folders and documents are organized in a hierachy like
+ * Each dms has its own database for meta data and a data store for document
+ * content. Both must be specified when creating a new instance of this class.
+ * All folders and documents are organized in a hierachy like
  * a regular file system starting with a {@link $rootFolderID}
  *
  * This class does not enforce any access rights on documents and folders
diff --git a/LetoDMS_Core/inc.ClassDocument.php b/LetoDMS_Core/inc.ClassDocument.php
index 21c815799..a1e23f8ee 100644
--- a/LetoDMS_Core/inc.ClassDocument.php
+++ b/LetoDMS_Core/inc.ClassDocument.php
@@ -25,6 +25,16 @@ define("S_EXPIRED",  -3);
 /**
  * Class to represent a document in the document management system
  *
+ * A document in LetoDMS is similar to files in a regular file system.
+ * Documents may have any number of content elements
+ * ({@link LetoDMS_Core_DocumentContent}). These content elements are often
+ * called versions ordered in a timely manner. The most recent content element
+ * is the current version.
+ *
+ * Documents can be linked to other documents and can have attached files.
+ * The document content can be anything that can be stored in a regular
+ * file.
+ *
  * @category   DMS
  * @package    LetoDMS_Core
  * @author     Markus Westphal, Malcolm Cowe, Matteo Lucarelli,
diff --git a/LetoDMS_Core/inc.ClassFolder.php b/LetoDMS_Core/inc.ClassFolder.php
index 274dc6a61..5ed715d24 100644
--- a/LetoDMS_Core/inc.ClassFolder.php
+++ b/LetoDMS_Core/inc.ClassFolder.php
@@ -15,6 +15,10 @@
 /**
  * Class to represent a folder in the document management system
  *
+ * A folder in LetoDMS is equivalent to a directory in a regular file
+ * system. It can contain further subfolders and documents. Each folder
+ * has a single parent except for the root folder which has no parent.
+ *
  * @category   DMS
  * @package    LetoDMS_Core
  * @version    @version@