Help Please

[ecchi]

This is driving me mad for three reasons:
1) I should know this, but I've bloody forgotten.
2) I should be able to find the answer on Google, but I just spent half an hour trying with no luck.
3) G mentioned this in a post some time ago, but I cannot find that post.

So hopefully here someone can give me the answer.

When you use the head tags (H1 H2 etc) you get a gap below the text. However you can control the size of this gap, and you can loose it altogether. But I cannot remember how. Do can someone please remind me of the HTML.

Thanks.