Different page layouts

Hey you guys (and girls)

I just started a new website with wordpress for the first time and got some issues with the layouts

for example, you got the categories witget, when i make a new page and put this widget in it, then by going to the menubar and clicking on that page gives me a layout with just a list of all categories.

And then, when i click on a categorie, i get all videos in that categorie, but they got their own postbox-class, while the categoriespage they just got the entry-class.

The reason i ask is because on a different page the videos are shown in the entryclass and when i go to the page-file and change the code, also the categories-page has the postbox class, that offcourse is not what i want.

So, where can i see those different types, cause evertything else in the code seems just the same.