when do you call an email a SPAM?

Wow! That is a really good question. Personally I dont know the specifics. I always assumed they were like the same thing with only slight differences, for example I thought that SPAM included such things as virues. Some of the other responders here seem to be very knowledgeable, enough to give you a very good and detailed answer. Things like that definitely help the rest of us and so I thank them for answering but also I thank you for asking the question to begin with. It was a great question that i'm sure people have asked themselves many times over

whenever the number of emails increases in number and many of them belong to unknown ids then i take this as spam attack.you

Mostly Junk mail and Spam are used as one thing.. but Spam is any emails that you may receive and you did not ask for it or have an interest in receiving it, even if you know the sender it may be a spam because you may be exchanging emails with a mylot member regarding the mylot site and he "Spams" you with links to other sites without asking your permission to send them or without you showing an interest in receiving such links.. Bulk mail is sometimes a reference to all newsletters subscriptions and groups bulletins, etc that you accepted to receive but not of a great interest to you. But usually the term "bulk mail" is used to just describe spam as explained above.