Question: Sometimes I see the search bar inside already present text "Search ..." and that once clicked is replaced by what the user wants to write:

Which is a better UX design?

a) leave the search bar empty and clean?

b) insert the text "Search ..."?

Answer: 
Use a prompt
============

I've tested this in ecomm and enterprise contexts in the past and providing some kind of "placeholder prompt" text has always been the winner. An empty field doesn't capture the same level of engagement.

As with all things, test with your users to validate.
As long as it's an obvious input field with the word “Search” and/or the icon either within the field or as a label for the field you should be fine.

Facebook had some problems with this when they implemented graph search. Fastco wrote an [interesting article](http://www.fastcodesign.com/1673209/how-facebook-fixed-the-biggest-design-flaw-in-graph-search) about their research.

[![enter image description here](https://i.stack.imgur.com/3qU3x.jpg)](https://i.stack.imgur.com/3qU3x.jpg)