Step plugins can be added to workflows as either Node Steps or Workflow Steps.
See: Jobs - Workflow Steps.
Workflow Step Plugins are triggered once during the workflow execution, and have access to the execution context, and list of Node targets.
Node Step Plugins are triggered for each target Node during the workflow execution. They are responsible for operating on a particular node.
To learn how to develop your own Workflow Step plugin see Plugin Developer Guide - Workflow Step Plugin.