What is it?: Those nifty bolded links with the tiny icons -- to someone's journal, like mpoetess or to a community, like lj_dev.
How do I do it?:
For an individual journal, type <lj user=journalname>
For a community, type <lj comm=communityname>
(If the journal you want to add is a syndicated feed of someone's non-LJ journal, like officialgaiman, just type <lj user=journalname> and LJ will put the correct icon on it to show that it's a feed.)
What if I'm not sure if it's an individual journal or a community, and I get it wrong? WOE! What will happen?: Shh. The Livejournal software knows, and will actually fix it for you. But it helps the server load a bit to do it right in the first place, I suspect.
What else is nifty about those links that a lot of people don't realize?: When you click on the name, the link takes you to that person or community's main journal page. When you click on the little icon, the link takes you to the user info, so you can read the bio and/or community rules.
What does [Unknown LJ Tag] mean? What did I do wrong?: "Unknown LJ Tag" shows up when you type a journal link, community link, or LJ Cut link incorrectly. If you get [Unknown LJ Tag] it probably means that you have typed a - after the lj in "lj user" or "lj comm" when there should not be one, or you have left out the - in "lj-cut" where there should be one. Or possibly you included quote marks around the user or comm name. The only place you should use quote marks in any of the special LJ tags is when you're typing in the text="Text of my LJ Cut" part of an LJ Cut.
[You do not get "Unknown LJ Tag" when you have simply spelled the user/community name wrong, or when you've typed in the name of a journal that doesn't exist. In that case you'll get a pretty link that looks like it would work, but leads to a page saying that the journal/comm doesn't exist. Like boopidoppiedoo.]