Re-working the basic-concepts chapter

Author: kinow

src/basic-concepts.md

@@ -46,21 +46,42 @@ requirements are supported in `cwltool`, but it is not part of the
 detail in another section.
 ```
 
-## CWL objects model
+## Processing units
 
-TODO: reading this topic now in the browser, I wonder if perhaps it is not
-      too advanced for users? Maybe we can explain inputs, steps, outputs,
-      without going in detail about the CWL objects model?
+A CWL document can also be called a CWL workflow. The CWL document is a
+JSON or YAML file — or a combination of both JSON and YAML. One of the
+required attributes of the CWL document is the `class`. The `class`
+defines the type of processing unit defined in the CWL document.
 
-The diagram below shows terms used in the CWL specification {{ cwl_version }}
-modeled as objects. You do not need to learn all these objects and their
-concepts to get started with CWL. We will focus on only a few objects for now.
+There are four types of processing units defined in the CWL specification
+{{ cwl_version }}:
 
-```{image} https://www.commonwl.org/v1.2/cwl.svg
-:alt: CWL Objects model
-:align: center
+- Command-line tool;
+- Expression tool;
+- Workflow;
+- Operation.
+
+### Command-line tool
+
+### Expression tool
+
+### Workflow
+
+Besides having `class: Workflow`, the CWL document of a workflow processing
+unit must also have the `cwlVersion`, `inputs`, `outputs`, and `steps`. The
+steps of a workflow.
+
+```cwl
+cwlVersion: {{ cwl_version_text }}
+class: Workflow
+
+inputs: []
+steps: []
+outputs: []
 ```
 
+A workflow step can call a command-line tool or expression tool.
+
 First we have the `Process` object used for the processing units available
 in CWL. There are four types in CWL that extend the `Process` type:
 
@@ -92,7 +113,7 @@ digraph "CWL Inputs, Steps, and Outputs" {
 }
 ```
 
-### Inputs and Outputs
+## Inputs and Outputs
 
 CWL has roots in "make" and many similar tools that determine order of
 execution based on dependencies between tasks. However, unlike "make", CWL
@@ -103,7 +124,7 @@ scalability: tools and workflows described with CWL can transparently leverage
 technologies such as Docker and be used with CWL implementations from different
 vendors.
 
-### Steps
+## Steps
 
 TODO: add info about steps (there must be some info in the old user guide?)