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: 
I would say that having "Search" as a placeholder or having a blank search input amounts to pretty much the same thing.

What I do as a pattern is using that placeholder to help the user by suggesting what to search, as in if you're doing a grocery site then just say "Search for vegetables, meat, fish..." according to what you formulate to be the most obvious search topics.

Also there's a lot of UI's that use the exact same design pattern (icon/input styling) for search inputs and filter inputs so if you have both you might want to use the placeholder to make the difference more obvious.
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)