Is junk mail the same as bulk mail?

I'm a bit concerned that I'm missing e-mails as sometimes it said it is downloading 6 messages for example and only 3 seem to appear. I don't have a junk e-mail folder to check. I have bulk e-mail which are kind of junk, but do any go anywhere else? Someone said on another discussion go to mail - preferences - enable junk mail filtering. This says that I have junk mail filtering. But when I tried to tick it to move to a junk mail folder it said 'Depending on how you have set your Mailbox Behaviors preferences in the Accounts pane of Mail Preferences, messages in the Junk mailbox might be deleted automatically.' When I look at this option the 'store junk mail on the server' option is unchecked. Does this mean that some messages are being deleted and not stored. I do get some messages which mail tells me it has designated as junk and asks me to confirm.

I just don't want to be losing important things. What is the best way to have this all set up?

Thank you, Rachel