Script Task Vs User Input Task

Did you attend the Appian academy? I think this is covered.

https://docs.appian.com/suite/help/21.2/Configuring_the_Script_Task.html

docs.appian.com/.../Configuring_the_User_Input_Task.html

1.  User input task node allows a designer to associate an interface to an user attended activity.
For detailed information about how to design interfaces, including how to enable user interactions, how to save and validate user inputs, and how to submit user inputs to process, see the Interface Object page. 

2.  The Script Task is used to perform an automated activity (which does not require a user to complete a form). It is typically used to execute an expression or rule and capture it in the process variable and use it further in a process. 

Conceptually ALL steps/nodes/smart services in a Process Model are "tasks". But it's only a "User Input Task" that is used to present a form/user interface to a User for, you've guessed it, "input" from that User! All other "tasks" have either very concrete functionality (e.g. "Write to Datastore") or the more abstract "Script Task" (which can invoke a wide range of  functionality in the rule!, fn! or a! domains)