Updated Concepts (markdown)

Concepts.md

@@ -21,6 +21,25 @@ A part entity have many fields, which can be used to describe it better. Most of
 * **Mass**: The mass of a single piece of this part (so of a single transistor). Given in grams.
 * **Internal Part number** (IPN): Each part is automatically assigned an numerical ID which identifies a part in the database. This ID depends on when a part was created and can not be changed. If you want to assign your own unique identifiers, or sync parts identifiers with the identifiers of another database you can use this field.
 
+## Core data
+### Category
+
+A category is used to group parts logically by their function (e.g. all NPN transistors would be put in a "NPN-Transistors" category).
+Categories are hierarchical structures meaning that you can create logical trees to group categories together. A possible category tree could look like this:
+
+* Active Components
+    * Transistors
+        * BJTs
+            * NPN
+            * PNP
+    * ICs
+        * Logic ICs
+        * MCUs
+* Passive Components
+    * Capacitors
+    * Resitors
+
+
 ## Attachments
 ### Attachment
 An attachment is an file that can be associated with another entity (like a Part, Storelocation, User, etc.). This could for example be a datasheet in a Part, the logo of a vendor or some CAD drawing of a footprint.