"What is an execution?"

A pointer to where you are in the process instance currently.

"What is the difference between an execution and a process instance?"

A process instance == a collection of executions, structured as a tree and stored relationally

"What is the difference between an activity, a task and a job?"

Activity = a step in the process, typically rounded rectangle in BPMN

task = something a user/system has to do

job = a timer or an async continutation of a process

摘自 术语定义