docs: add draft on operators

docs/operators.md

@@ -1,7 +1,22 @@
 # Operators
 
 Operators are used here as a general term of callable code blocks that operate
-on data. In Airflow, operators are used to define tasks in a DAG. In
-`fasthep-flow`, operators are used to define stages in a workflow. The operators
-are defined in the YAML file, and then mapped to Airflow operators when the DAG
-is generated.
+on data. In Airflow, operators are used to define tasks in a
+[DAG](https://en.wikipedia.org/wiki/Directed_acyclic_graph). In `fasthep-flow`,
+operators are used to define stages in a workflow. The operators are defined in
+the YAML file, and then mapped to Airflow operators when the DAG is generated.
+One `fasthep-flow` operator can map to multiple Airflow operators.
+
+## Operator Types
+
+There are four types of operators:
+
+1. **Data Input**: These are a special set that does not require any input data,
+   and instead generates data. These are used to start a workflow.
+2. **Data Output**: These are a special set that does not require any output
+   data, and instead consumes data. These are used to end a workflow.
+3. **Data Transform**: These are the most common operators, and are used to
+   transform data or add data to the workflow.
+4. **Filter**: These are used to filter data. They are similar to data transform
+   operators, but instead of adding data, they restrict part of the data to
+   continue in the workflow.