I hope this will help you to understand the actions in Outsystem.
1. Server Action
=> set as a function and use it direclty in expression
=> encapsulate the logic and resure in other actions
=> strong dependency from consumer to producer module in tightly-coupled way
2. Screen Action
=> run logic when user interacts with screen
=> add screen action in web block as well
3. Client Action
=> Run logic on client side/user device
=> create in logic tab (encapsulate logic for reuse in several screen) and screen (when user interact with screen) as well
=> e.g like button click, link click, etc
4. Service action
=> REST based remote call to another process.
=> week dependency from consumer to producer module in loosely-coupled way
Hope you got the answer here. Below is a example of actions in attached image
Thanks
Prince