Exploring Processes Elements
A Process Thread can be broken into multiple ProcessElements. Process element is defined as the basic atomic component in any process structure.

A process element is mostly a step into the process. The process elements can be custom set to do almost anything by configuring it appropriately.
Activity Process Element
When a process element is configured to do some specific tasks; such as sending an email or saving a record, it is called Activity Process Element.

An Activity Process Element can call a service, process, transaction or a rule in the Rules Engine. By effectively getting tied up with the Rules Engine, a Process Element can accomplish almost anything.

Gateway Process Element
Gateway Process Element defines the path that the process flow should take. For example, an OR-Gateway determines which flow path the process should continue after evaluation of the condition. Similarly, a FOR-EACH – Gateway loop, through each object in a collection of objects, will execute a job for the object. Say for example it can loop through database records and save each record to the database.
Event Process Element
This element generally indicates an event within a process flow. For example “Start Event Process Element” takes place when a process starts.