I like to think of it as boxes.
<div></div> is a box. Inside this box you can put something.
<div>It was the best of times...</div> You can put your site content in a box
<div>Home. Links. About Us. Contact Us.</div> You can put your Navigation in a box.
<div>Google Ads</div> You can have some cash flow
What can you do with this? Well with CSS you can tell each of these elements where to go. You can use absolute positioning to put everything exactly to the pixel where you want it. This isn't *ideal* CSS design as it's pretty static like that.
I like relative positions

You can tell things to float:right or float:left and the <div> will flush with that side of the screen! And if you have multiple <div> in a row you can flush all of them to the left and create columns! Provided, of course, that you have ample screen width.
The rest of <div> is lots of details. You can specify width, height, font, color, borders, etc. It's like tables like that, but CSS has a great advantage. You can store these parameters of CSS in a .css file and tell all the <p> tags to be x Font and x color. So anytime you enclose a paragraph in <p> tags, it'll take those properties! And want to change your mind later? Just change that one little line in your .css and all your <p> tags get changed
I could go on, but get yourself a resource and take a look at examples. I wouldn't look too much at other sites (unless you see something you want to learn) because often other sites use sloppy code :belch: