Hi @SHANMUGA PRIYA 

Server Action:-

 Server Actions are Actions that are always executed on the server, both in Web and Mobile. Since Web Apps run on the server in their entirety, this is the only available type of Action for a Web App. When a Mobile App calls a Server Action, it needs to be online (since it must reach the server to execute the Action). Use Server Actions in Web Apps whenever you need to abstract some business logic or core logic or domain-specific logic. Use Server Actions in Mobile Apps only if you cannot avoid it (e.g. when syncing data), as it will incur a performance penalty (for calling the server and waiting for it to respond back). 

Client-Action:-

Client Actions are the Mobile equivalent to Server Actions. They run on the device, not on a server. Client Actions are therefore only available for Mobile Apps. Their use, like Server Actions, is to abstract logic.

Since OutSystems 11, there is one more action added that is called  Service Actions. These are very much like Server Actions, but share some properties with REST Methods (perhaps yet another type of Action), in that calling them only creates a "weak" Reference to the Module it's in (instead of the normal "strong" Reference). But if you've just started OutSystems, I'd forget about them for now.

Thanks and Regards

Vednarayan Saket

Client Actions:

1. Client actions are executed in client side like javascript.
2. Client actions are only available in Mobile apps and Reactive web apps.
3. You can use client actions in expressions if its function property is set to true.
4. Mostly used for validations and widget event handlers.
5. Use your client actions light weight.

Server Actions:

1. Server actions will be executed in server side like database activities.
2. Server actions are available in both Mobile and Web apps.
3. Use server actions in mobile apps only if it is necessary/unavoidable.
4. You cannot use server actions in expressions even if it's function property is set to true.
5. Using frequent server actions may affect the performance. 

Client Action : An Action cannot be exposed when:

• It has a parameter that is defined using an Entity/Structure that is not exposed.
• It has a parameter that is defined using an Entity/Structure that is reused from another module.

In case the execution of the Producer and Consumer modules are under different User Providers, the modules have different sessions. In this case, variables associated to the session can hold different values between modules.

Server Action : 

An Action cannot be exposed when:

• It has a parameter that is defined using an Entity/Structure that is not exposed.
• It has a parameter that is defined using an Entity/Structure that is reused from another module.

In case the execution of the Producer and Consumer modules are under different User Providers, the modules have different sessions. In this case, variables associated to the session can hold different values between modules.

some of the link to ref:  https://success.outsystems.com/Documentation/10/Developing_an_Application/Implement_Application_Logic/Actions_in_Web_Applications?_gl=1*11gm88l*_ga*MjYyODk0MjgzLjE2MzcyODg3MzU.*_ga_ZD4DTMHWR2*MTYzNzY1OTU0Ny44LjEuMTYzNzY2MjMyNi40MQ..#Screen_Actions