I heard much more about Joomla on the web. But I am confused with. What can Joomla do as CMS. and what about the security?
Joomla is an opensource Framwork, there are some built in joomla Templates which you can use to build your CMS site. It reduces you task around 60%. Because the templates provide u dynamic ability to enhance and maintain or update according to your wish within a limited time. Have a try man.
Joomla does have its share of security bulletins and upgrades. One great place to search through is secunia dot com (can't post links because of post count.) Generally Joomla is fine if you keep it updated though. You also have to keep track of third-party add-ons because they may also have their own security problems. Just keep everything up to date and you will be fine. If an add-on that you are using is no longer supported then get rid of it.
My experience with Joomla security threats has mostly been through unsupported third party components. If you're running your Joomla site through a linux server, and you keep it locked down tight, you shouldn't have many issues. 1.0.13 is stable, and most of the hacks and exploits have been corrected. It's not a perfect system, but you can only do so much. Always back up your data bases, and your content, use a reliable service provider, and you should be fine.