What is CMS?
CMS (Content Management Systems) are ways in which to design websites. The two most popular and commonly-used are Joomla! and WordPress. Like every product out there, there are designers who swear that one is better than the other... that one is horrible while one is best. I don't think either is necessarily better than the other because it all depends on what you are needing for your site.
CMS typically are "open source". Websites done in html language typically can only be changed/tweaked on a specific computer with the files. An open source website doesn't have that limitation. Any computer (and a lot of mobile devices) that has internet access can make changes to the site. Users can be set up so different people can access and make changes to different parts of the sites.
People will argue that search engines like WordPress more than Joomla!. That used to more true, as the blogging aspect of WordPress is very friendly for search enginges. However, with each version of Joomla!, that becomes less true. Joomla! has made great strides in SEO optimization in it's native installations, and the add ons have increased dramatically. I no longer believe it's accurate to say that WordPress is better for SEO.
WordPress was first developed as a blogging system. It has since become "blogging software trying to do websites". This is not necessarily a bad thing, it just means that you are limited in some ways in regards to desiging and what you can do. WordPress is easy for the novice user to use. You can go in and create posts and pages with relative ease.
However, it's more limiting of the 2 CMS's that I am talking about here. It works best on blogs and simpler sites.
You can customize the layout as needed, however, some of the more "simple" design elements, such as different images or information appearing in the margin on different pages, needs php programming knowledge to do if the theme wasn't set up to accommodate it originally.
Joomla! has always been website designing software. It does have a slightly higher learning curve than WordPress for the average user, but it has a lot more functions and capabilities. Both Joomla! and WordPress have a plethora of themes available and add-on products that can increase their offerings. But Joomla! has more of that strength "out of the box". If you want different images to show up for each page, no problem. If you want a different template to appear on one menu item, no problem.
Which do I design in?
Primarily, I design in Joomla!. The reason for this is three-fold. First, I'm just most comfortable with it and my experience tells it's the most flexible. Two, even a client who says that they want just a simple informational website will ultimately come to you with a request that makes the site more complex. Joomla! allows the site to be "on the ready" for requests such as these, and allows for higher customer satisfaction. Finally, as a designer, Joomla! makes some of the design elements much simpler to implement.
Overall, the control panel for each CMS is such that it would be simple enough for a novice user to use with a cheat-sheet or little instruction.
Once you describe to me the type of site you are looking for, I can make a recommendation on the type of CMS that would be best for you. If you are set on WordPress, I will tell you if the design or features you able to be done in WordPress.