I assume the spiders to be so-called "bots", automated programs that browse the internet for various purposes (members, lurkers and "bots" are listed in that order in several other forum boards I know,). As for the second question, I was wondering too. Sometimes I appear in the list, sometimes not... Go figure.
A spider is nothing more than a computer program that follows certain links on the web and gathers information and creates a copy of all the visited pages. This will index the downloaded pages so that it will show up quickly in the SERPs (search engine results page). I don't think it can detect the # of people online tho.