Nesting, Grouping CSS Selectors

Honestly just looking for some clarification as for some reason this concept seems to be giving me a bit of a hassle.

Am I correct in assuming that when nesting CSS selectors that it should be listed from most specific to least? If it is not a nested element but only a group of elements does is matter if it is in order of specificity?

Ex. if I was asked to Set the #banner .content h1 width to 400 pixels. as I am in the Box Model section of the CSS course, it would be written like:

#banner .content h1 {
width: 400px;
}

#ID (very specific) .class (less specific) h1 (tag which would not be specific at all).

Also, the difference between grouping and nesting: The way I read it is grouping is if you want a group of elements regardless of where they are in the HTML page to be formatted a particular way you would do that by separating each element with a comma and keep them on one line. But nesting is where you only want to format a particular section of the document which is controlled as in the situation show in the above code.

Not really sure why this is giving me pause but before I get too deep I just want to make sure I am rationalizing this correctly in my head.

Thanks in advance