User input data validation should occur on client side.
Then for each transaction there is no need to contact with 
server thereby  transactions decreases and efficiency 
increases.

user input data validation can occur both side means client 
side as well as server side. but generally validated on 
client side because in this validation no any sever 
resources used for validation. So it provides  less 
overhead on server and application performance become fast 
rather than server side. if we validate user input data on 
server side every time data check on server so the 
performance of application become slow down.

Client side validation such as javascript can be bypassed.
It is not secured at all. But it can be used to provide
quick feedback to the users. For example, if the user enters
invalid username and jump to another field, show an error
message quickly with client side validation.

Sever side validation protect against  against the malicious
user. Using both techniques can provide secrue, quicker
validation and compatibility .