When you submit your e-mail thorough a web form, let say:
Which means, no Internet connection needed, it’s fast since you don’t have to wait for the page to be refreshed to see the validiation..
With Server side validation, everything is the opposite. I would say client side validation is safer.
Client side validation means the validation of a piece of data is executed on the client’s machine, while the server side validation means that the data is sent to server side first, then the validation is carried out.
This would need some examples to put the terms in context. Client side validation, for most of the software that runs on a PC without network or internet connection, if there is any validation, there is no distinguish of “client” or “server”, because they are the same (the PC).