CentOS has all the major desktop environments (Gnome, KDE, Xfce, etc.) and it's impossible for me to tell from my computer which DE you are running on your computer.
The default DE is Gnome, shown in this screenshot: http://upload.wikimedia.org/wikipedi...CentOS_6.0.png
You can easily install Gnome DE in Ubuntu using the Software Center. However
Ubuntu has newer software than CentOS, so it will be a newer version of Gnome, which will look a bit different. (CentOS uses older software that is very well-tested and stable.) The closest you are probably going to get to a CentOS 6.x "look and feel" on Ubuntu are either the MATE or Xfce DE's.
But why not just use CentOS---it's great!