When posting an entry or comment, LiveJournal normally adds line breaks any time you hit the enter or return key, and turns URLs into links. However, you can code HTML manually with the "Disable Auto-Formatting" option on comment forms and the HTML editor on the Post an Entry
In addition, <lj-raw>
disables auto-formatting for all or part of your entry, comment, or user bio.
<lj-raw> unformatted text </lj-raw>