Okay, know how when you get some programs from the Internet, like Firefox or music players and such and they all you to change the “skin”? Themes are like that but not.
The simple explanation is that Themes use CSS and modular template files that act like building blocks to create your page. Everything is styled with CSS, but the page is divided up into blocks.
In the Default Theme that comes with WordPress, when you view the mutli-post pages like the font page, searches, categories, archives and such, you will see a list of posts in the content section and a sidebar for navigation around the site. But click on any of those posts and the sidebar disappears. The designer felt that people should be able to read a page without the clutter of a sidebar, so he “dropped that block” from that page.
You can change any of the “blocks” to look different and respond differently based upon some PHP questions and answers. Questions like, if this post is in a category about blondes, then make the page feature a blonde woman standing next to a car in the header. If this post is in a category about redheads, then feature a header with a redhead sitting on a bale of hay. Or much more complicated questions and answers, but I’m sure I’ve titilated enough folks.
Everything in those “blocks” called template files are styled with CSS with are the “styles” (Cascading Style Sheets).
Here are the more complicated answers:
https://codex.www.ads-software.com/Using_Themes
https://codex.www.ads-software.com/First_Steps_With_WordPress
https://codex.www.ads-software.com/WordPress_Lessons