What is the app which let user to find by finding application and files with key combinations? I am not a mac power user and when ever I need to find an application or file, I open Finder and look for it. Couple of weeks ago, I saw someone using some keyboard shortcut, which opened a dialog box, where as he enters the name, the application showed the files matching the word he is searching. Is this a mac power tool or is this a different app?

Thanks
Spotlight is a part of OS X. The default keyboard shortcut is, I think, Command-Space for the Spotlight menu entry and Option-Command-Space for the Spotlight field in a Finder window. (Clarified after Chris' answer.)

There is also a third party application named Quicksilver.