A content management system is an application that is accessed with a webbrowser (mozilla firefox, Opera, konqueror etc) over a network like Internet or through the intranet. We actually call them webapplications.and thats where we start talking about webapplication security.
We use these webaplications for webmails, online retail sales, online auctions, wikis, weblogs, discussion boards(forums) and alot of others.
Content Management System simplifies the work of the administrators on a site, when it comes to stuff like editing processes, creating, translations, publishing, archiving and alot more better services.
Errors and bugs found are released everyday including hacked frameworks, new and zero day exploits, and people are made aware of them and patches released. To learn more about webapplication security, you can check www.webappsec.org, a site i always visit several times a day. Others are like www.owasp.org, www.spidynamics.com, www.acunetix.com, www.cgisecurity.com
Most of the Content Management Softwares i know of, are opensource, so free for anybody who wants to have a website which really sings..........