In order to get DVD's to play, you will need libdvdcss.
Go to a terminal and log in as root by typing:
then type in your password and hit enter
Then install the file using the following command.
yum install http://download.videolan.org/pub/libdvdcss/1.2.9/rpm/libdvdcss2-1.2.9-1.i386.rpm (This works whether you have 32 bit or 64 bit installed)
Installing this file is illegal in some countries, but, perfectly legal in others. You can read about the legalities at http://www.videolan.org/developers/libdvdcss.html if you so wish.
If you want to install media codecs or non-free media players you will have to have the non-free Centos repos activated in your Centos install. You can find instructions for setting up these repos here: http://rpmfusion.org/Configuration . When installing the rpmfusion repos, make sure you install both the free and non-free versions.
As far as a media player, you can search your package management app in Centos, which is PackageKit, the same as it is in RedHat and Fedora. You can find it in the Administration part of your main menu.
Just as you installed the above file, you can install any app in Centos by becoming root in a terminal and typing:
yum install <packagename> OR you can choose the app in package management and install it that way.
I prefer VLC as my media player, some prefer mplayer or xine. I'm sure all are available, searching PackageKit should confirm their presence. If you use the search term "multimedia" and search by definition instead of package name, a host of different media players should come up. Read the details of each and decide which ones suit your needs. For me, VLC does it all.
yum install vlc
If you have any more questions, just ask. Hopefully the instructions above will work for you on first try, but, if you encounter any problems, we can help you sort them out.