If you're reading this, you've just seen the feature in action. It's called <lj-cut>. It lets you divide an entry so that only part of it shows up initially, and you have to click to see the rest.
I'm sure you can see the many uses this could have, ranging from giving a summary of a long entry, with a link to the rest, or telling people about pictures while letting them click to actually see them, so that friends pages and such load faster.
So basically, the way you use it is to just put <lj-cut> where you want the cut to be. To make it look nice, put it on the first line of your second paragraph (or wherever your long text starts) so you don't get extra breaks in there.
Over all, I'm thinking <lj-cut> is a very nifty feature. ;-)
