I thought I'd post this in case posters haven't used lj cuts (and, personally, I find LJ's search and FAQ clunky to use, so this may help others who've given up on it).
An lj cut allows you to hide all or part of your entry behind a link. The part you "cut" is hidden on your journal and on Friends pages, but you can still see the full entry on the "Read Comments" page, the Preview Entry button, and the view entry link on the Post an Entry confirmation page.
When you are writing an entry, on the upper right hand corner of the text entry box you type in, you will see "Rich Text" and "HTML."
*If you use Rich Text : After you type your entry, highlight the text you want to hide under a cut, then press the LJ CUT button--the button on the tool bar at the top of the entry box that looks like three lines with a squiggle under it. A dialog box pops up with "Read more..." text, which you can leave or replace with text you prefer. Hit "ok" and a link is automatically created. You've probably seen the "Read More" links in people's entries. Once you post your entry, all a reader has to do is click that link to see your entire entry.
* If you use HTML editor: Type <lj-cut text="Read more--or whatever text you prefer"> before the text you want to hide. If you want to start showing text again, type </lj-cut> after the text you want to hide.
That's it! It takes longer to explain than to do!