[waterside81]

That's interesting. I've always wondered if some people had conventions on when to use padding and when to use margin, since you can almost always accomplish the desired effect either way.

[meeker]

Except that adjacent vertical margins collapse on each other, so the space is the larger of the 2 margin values not the sum.

more info: http://reference.sitepoint.com/css/collapsingmargins

This is not true of padding (unless you count a bug in IE8 where percentage-based padding-bottom collapses on the margin-bottom)

[Facens]

If you want to be modular, you must have conventions. This is to prevent double spacing when you put two elements one next to another without additional styling. I also use to have ELEMENT > *:first-child {padding-top: 0} on containers, so that I can control the padding of the container and without having to consider what element I place into it. There's a lot of best practices I made part of my workflow, perhaps I should write a blog post about it.

[joahua]

You can't really. CSS backgrounds are the main reason you need to use padding (instead of margin): margins exist outside an element, while padding expands within the box itself (beyond the content, but permitting the background to remain visible).

To visualize it, http://hicksdesign.co.uk/boxmodel/ is probably the most oft-cited example - certainly the one I find most helpful.

[Facens]

Ok, I was talking about transparent elements, where in fact you have complete choice (at least if you have no borders too).

[mikeklaas]

In my experience, that is almost never the case, except when you're faking margin by using padding.

In that case, use margin.