Add information about the Merlin DAG

Define the important terms of the DAG.

Author: mikemckiernan

docs/source/about-dag.md

@@ -0,0 +1,31 @@
+# About the Merlin Directed Acyclic Graph
+
+Merlin uses a directed acyclic graph (DAG) to represent operations on data such as filtering or bucketing and to represent operations in a recommender system such as creating an ensemble or filtering candidate items during inference.
+
+Understanding the Merlin DAG is helpful if you want to develop your own operator (Op) or building a recommender system with Merlin.
+
+## DAG Terminology
+
+node
+: A node in the DAG is a group of columns and at least one _operator_.
+  The columns are specified with a _column selector_.
+
+column selector
+: A column selector specifies the columns to select from a dataset using column names or _tags_.
+
+operator
+: An operator performs a transformation on data and return a new _node_.
+  The data is identified by the _column selector_.
+  Some simple operators like `+` and `-` add or remove columns.
+  More complex operations are applied by shifting the operators onto the column selector with the `>>` notation.
+
+schema
+: A Merlin schema is metadata that describes the columns in a dataset.
+  Each column has its own schema that identifies the column name and can specify _tags_ and properties.
+
+tag
+: A Merlin tag categorizes information about a column.
+  Adding a tag to a column enables you to select columns for operations by tag rather than name.
+
+  For example, you can add the `USER_ID` and `ITEM_ID` tags to columns.
+  Modeling and inference operations can use that information to act accordingly on the dataset.

docs/source/toc.yaml

@@ -46,5 +46,7 @@ subtrees:
                     title: Deploy the HugeCTR Model with Triton
                   - file: examples/scaling-criteo/04-Triton-Inference-with-Merlin-Models-TensorFlow.ipynb
                     title: Deploy the TensorFlow Model with Triton
+      - file: about-dag.md
+        title: Merlin DAG
       - file: containers.rst
       - file: support_matrix/index.rst