Activities 
An activity is kind of work that is performed within a business process. An activity can be atomic or non-atomic (compound). A compound activity is the sub-process.
The following activities are available in the process composer:
Human Activity
A human activity is an activity in the process that is performed by a person. A task needs to be assigned to the human activity. When the process is at the point of the human activity, the assigned task is shown, for example, in the universal worklist, and can be executed by a person. For the task, input and output data mappings need to be defined.
More information: Modeling Human Activities
Automated Activity
An automated activity is work that is performed by a system. To make the automated activity work, a service interface definition and an operation need to be assigned to the automated activity. In addition the input and output data mappings need to be defined.
More information: Modeling Automated Activities, Managing Processes and Tasks with the Process Desk
Sub-Process
A sub-process is a compound activity that is defined as a flow of other activities. Every process can be a sub-process in a main process. The sub-process has a start and an end event. The sub-process is displayed in the process model of the main process in a collapsed way.
More information: Working with Sub-Processes
Mapping Activity
A mapping activity can be used to transform complex data from data objects in the process context into more simple data. Rules set can be used as a transform in a mapping activity. To make the mapping activity work, the mappings between the data objects in the process context need to be defined.
More information: Transforming Complex Data to Simple Data with Mapping Activities, Working with Rules