An activity is a unit of work that is executed within a business process; it can be atomic or non-atomic (Compound activity, Call activity, or Subprocess). Activities can be of the Task, Call activity, or Subprocess type.
A task is the smallest atomic activity unit that can be included within a process. Usually, the performer of the task can be an end user (called human) using a UI-based application, a participating external service, or a generic set of business statements. Tasks have their local scope and can accept input parameters from their container and return output parameters.
A user task is used to model work that needs to be done by a human actor. When the process execution arrives at the user task node, a new task instance is created in the work list of the actor(s) or group(s) defined for this task (the Actors and Groups properties). Human tasks can transition to several different states and involve human stakeholders depending on the action taken on the...