I would describe a Content Management System (CMS) as a sort of mould; someone crafts the shape, anyone can pour in the melted ore, and that ore will take whatever shape it is put into. It is the same with a CMS - a theme is created and anyone, even without website design experience, can add the content. The difference to the mould analogy is that the content and the shape can change.
As you can guess, Concrete5 is the CMS that I recommend and that I would set up sites with. The reason for this is that it is easy to create themes for this CMS; consequently designing sites to work with it, or taking exisiting themes and adapting them to work with it, is a process acheivable without hours and hours of input.
If you have enquiries about using a CMS, would like your exisiting site to be adapted for use with a CMS, or want a site designed to work with the CMS, please contact me using the details above.