There's no single, universally-accepted, detailed definition of spam, which is one reason (out of many) why spam filters aren't perfect. Everyone pretty much agrees that the illegal or quasi-legal email that clutters your inbox is spam. No reasonable person will defend the V1agra ads, stock pump-and-dumps, or anything about millions of dollars that only you can take out of Nigeria.
Things get murkier when legitimate companies send out mass, unsolicited email. That's why most of these solicitors, unlike the obvious spammers, will honor your request to be taken off the mailing list.
But some people are overeager to declare anything mailed to more than five people spam. Email friends and relatives, recommending a film or restaurant, and there's a good possibility that someone will complain that you're spamming them. Your best bet is to send that person less email.
It's very much against netiquette to post anything remotely promotional on a Usenet newsgroup or an Internet forum. If there's someone policing the forum, your post will be taken down immediately. You can get away with it in some forums if you're a regular contributor, the message is very much connected with the forum's subject matter, and you preface your post with something like "Shameless Self Promotion."
There's another issue: Someone else might be sending spam that's spoofed to look like it's coming from you. For details, see Why Am I Sending Myself Spam?