A Workflow graph is a description of the case lifecycle consisting of a set of stages interconnected with transitions in a directed graph with one designated start stage (often referred to as the "Inbox") and at least one or more designated end stages.
Each Process has one and only one Workflow graph. All cases in the Process follow this graph and when cases are created they always start at the designated start stage.
Various users normally observe different stages and typically someone makes a first rough categorization and assessment and ensures the case fulfils the required criteria in order to make a transition to another stage.
See for more details.