By default auto-formatting is enabled. Auto-formatting does the following:
* Automatically inserts breaks between lines for you. If you hit enter to move down to the next line, that is how the post will appear on your journal.
* When linking to a site, some HTML will be created for you. For example, if you type http://www.insanejournal.com/ in an entry, it automatically becomes a link.
If you prefer to format posts yourself (for example, using to create a line break), then you should select the "don't auto-format" option. If you don't know what you want, you probably want to keep it unchecked.
Alternatively, you can disable auto-formatting using the IJ-specific tag . For more information on the tag, see http://www.insanejournal.com/support/faqbrowse.bml?faqid=49 .