Classes and divs, once and for all

Some helpful folks here have enlightened me to sorted out the main difference between classes and divs and how to use them. But I see something defined this way in the style sheet:

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Some helpful folks here have enlightened me to sorted out the main difference between classes and divs and how to use them. But I see something defined this way in the style sheet: and also . What is the difference between those and good old fashioned <#divname>? I've used <#divname .class name> to define a class inside a div, and I mostly get that. But what about the other variations?

Also, what is the point of defining a class inside of a div (such as <#divname .class name>)? I've tinkered around a bit and sometimes noticed a difference between defining a class in a div versus just having the class defined independently, but if someone can summarize it for my memories section that would be great. Thanks!