This is (a) a draft only, (b) horrendously long, and (c) based on burr86's suggestion of straightening/merging the comments FAQs, so feel free to tear apart or fold or otherwise mutilate this as necessary. :)
FAQ: What are comments? How do I enable/disable message boards?
[brief blurbly information on what the heck comments actually are, which I'm too lazy to write up now] Comments enable other people to respond to your entries (or to other comments associated with an entry).
The full version of your userinfo page (http://www.livejournal.com/userinfo.bml?user=exampleusername&mode=full) will keep track of the number of comments you have received and posted. The first number, Posted: X, represents the number of (non-anonymous comments) you have made in any journal. The second number, Received: Y, represents the number of comments that have been made in your journal.
Your journal (http://www.livejournal.com/users/exampleusername) and friends page ((http://www.livejournal.com/users/exampleusername/friends) will indicate the number of comments on any given entry. However, this number does not include screened comments.
ENABLING AND DISABLING COMMENTS
To enable or disable commenting in your journal, go to http://www.livejournal.com/editinfo.bml and find the option labeled "Enable message boards". Selecting this option will enable comments; de-selecting it will disable them. (Note that when you change settings on this page, you need to save the changes by clicking the button labeled "Save Changes" toward the bottom of the page.) Here you can also set commenting options for your journal:
* Who can reply to your entries (http://www.livejournal.com/support/faqbrowse.bml?faqid=23)
* Whose IP addresses, if any, you want to log (http://www.livejournal.com/support/faqbrowse.bml?faqid=66)
* Which comments you want screened by default (http://www.livejournal.com/support/faqbrowse.bml?faqid=134)
You can also choose to receive an e-mail notification anytime someone replies to one of your entries or comments. To do this, check the box next to "Get message board replies". You can also select whether you would like to receive plain text e-mails or HTML-enhanced ones. Paid, Permanent, and Early Adopter accounts can also receive notifications for comments that they write, by selecting the "Get comments you post e-mailed to you" option. This option is disabled for Free accounts; however, e-mail notifications for received comments is available to all account types.
If you have selected the option to receive comment e-mail notifications, but are still not receiving them, see http://www.livejournal.com/support/faqbrowse.bml?faqid=115 for information on why this problem may occur and how to resolve it.
It is also possible to disable commenting, disable comment notification e-mails, or change the screening options of an individual entry. (Disabling commenting, either for the entire journal or for a specific entry, will also hide any existing comments.) The full mode of the web interface (http://www.livejournal.com/update.bml?mode=full) allows you to change these settings as desired; it is also possible to edit entries to change one of the comment options. Many clients available for download include the ability to set security levels for journal entries. For more information on your preferred client, consult its documentation or its menu options. See http://www.livejournal.com/support/faqbrowse.bml?faqid=158 for information on downloading a client.
It is not possible to selectively enable comments or notification e-mails if they have been globally disabled for that journal.
Comments are organized into threads; it is possible to reply directly to another comment, rather than just replying to the entry itself. A comment is considered a child of the parent comment (or entry) it is a direct reply to;
The "thread" link associated with a particular comment will display a single comment thread: that particular comment, plus any descendents (children, or children of children). The "parent" link does the same thing starting one level up: the parent of that particular comment, plus any descendents of the parent comment. This allows you to view, or link to, a single thread of comments. (Top-level comments, which are direct replies to the entry, do not have the "parent" link, as the entry itself is the parent.)
When there are at least 50 comments on a page, the comment threads will collapse, so lower-level child comments display as a link rather than the full comment. This behaviour is automatic, and thread collapse cannot be either prevented or triggered sooner.
For information on deleting, screening, or freezing individual comments or comment threads, please see http://www.livejournal.com/support/faqbrowse.bml?faqid=15