Functions fired on client side or server side

Functions fired on client side or server side

Hi:

As we know, for a database driven online application, usually we write code and the code will run back on the server, but sometime we use script (such as: javascript) which can run on client side.

I am a little bit fuzzy on Deluge, I noticed from Deluge help: 'alert' can be used only in 'validate', 'on load', 'on user input' and on click actions (so I suppose it runs on client side).

How can we choose or tell if we want to run the code on client side or server side?
(for example, some validation can run on client side, but some complicate validations have to be run on the server).

Thanks.