According to BPMN 2.0 specification, an activity represents “work that a company or organization performs using business processes.”
In a BPMN process diagram, an activity is represented with a rounded-corner rectangle and named according to the kind of work, which has to be done.
BPMN defines two main types of activities: atomic and compounded.
An Atomic activity is defined as a task and represents an individual action in a process that doesn’t need to be broken down any further.
A Compounded activity is defined as a sub-process and represents work in a process that can be broken down to a finer level of process model detail. A sub-process can be represented in an expanded view that shows its details or in a collapsed view that hides its details.
A special type of an activity is a Call Activity, which references a reused globally defined task or sub-process. A Call Activity is represented with a thick border.
Beside these main types of activities, BPMN 2.0 defines different behaviors and properties of activities, which are visually annotated with standardized symbols, for example: a repeatable activity, a user performed task, an automated task and a transactional sub-process.