BML is the language used to serve pages to users that they don't manage themselves. To get an idea of what BML is, the following is a quote from the BML manual:
BML is a server-side markup language that lets you define your own BML blocks and use them as templates within your BML pages. Your templates don't even have to be static. Because BML pages are converted to HTML on the server when users request them, this also enables you to embed live code within your BML pages, just like a CGI script.
|--The Better Markup Language|
With LiveJournal, we used BML because it makes it easier for us to write the layouts that comprise LiveJournal, without requiring frequent changes or rewrites on our part to come up with a customized look and feel. In the next section you will read how to make the necessary additions to the LiveJournal schemes to tailor your LiveJournal installation to your exact needs.
For more information on BML, please refer to BML.