Project Plan: The project plan includes all the plans present in a project such as work breakdown structure, schedule, resource plan and cost plan.
Work breakdown structure: Defines what needs to be done in the project. Does not make statements about time, resources and costs. Tasks are grouped and structured in a meaningful way. A person responsible is defined for each (sub)task. The work breakdown structure documents the project structure with subtasks and work packages as a hierarchical diagram, table or even list.
Task: A work order under defined conditions and using specific resources.
Work package: A self-contained task that is not further subdivided in the work breakdown structure. The sum of all work packages results in the project.
Task: A task is a process element that represents self-contained tasks. A work package defines what is to be done, operations define the time sequences of the tasks.
A subtask, as the word implies, is a part of a task and can itself be broken down into subtasks.
Critical Path: The critical path determines the total duration of the project. It is the sequence of activities and milestones where there are the least or no buffers/reserves. Another definition: the path through the network with the maximum accumulated duration is called the critical path. If an activity that lies on the critical path takes more time than planned, this directly affects the project plan.