They represent atomic units of work done as part of a business process.
There are three types of tasks:
None: represents a generic task. This type of activity does not have an associated application; it is automatically executed by the engine and affects neither the relevant workflow data nor the application data.
Script: are those activities that do not require interaction by the user and will be executed by the Workflow engine.
User: represents a task executed by an user.
Below are examples of each one of these symbols: