Mambo is a Content Managment Software. If you have heard of PHP-Nuke its sort of like that. Mambo makes the Webmaster job alot easier due to you not having to manually code everything. It uses php/Mysql. I have to say its one of my personal favs.
Mambo is a PHP, mySQL and a CSM system...But I wouldn't highly recommend it cause somepeople say that there Mambo sites got hacked...The one I would recommened is LDU another CSm system, alot faster than PHPNuke and Mambo, easy to castomize and adding plugins. Can even create your own skin and code it for your site...There sercurity system is great, also they have just released there new version LDU v800 and LDU Weblog Edition. I say LDU is 10 times better than PHPNuke and 5 times better than Mambo. www.neocrome.net that site is also Powered by LDU.
Firstly its not CSM its CMS - Content Management System
Secondly the mambo sites that were hacked most likely had not patched their systems. Which is important no matter what CMS you use. Security holes are a given in any program they will always be there.
Thirdly when choosing a CMS you need to consider what you are going to use your site for and what you are comfortable using. Just because someone or a group likes a particular CMS doesn't mean it's the right one for you.
Finaly going back to security if you keep your site updated you will reduce the chance of it been hacked.