In the picture above see the icon of magnifying glass. I didn't have this icon before, It is indexing my files. Any idea what is it, I have maverick installed on 2 machines and both of them now have this icon.
When I click on it and search for some file, on the title bar it says "Tracker Search Tool"
It is really cool to search for files but I would like to know that if it is one of the default app, even though it was not there before nor I installed any app like this.
Tracker is a search engine and metadata store that indexes your files to do all sorts of interesting things. See this answer for some usage basics.
It hasn't been installed by default in Ubuntu since Ubuntu 9.04; you must have installed it inadvertently. If you'll not be using it, you can remove it by removing the "tracker" package.