Spam generically means unsolicited and inappropriate commercial messages invading either your private space, or a public forum.
Sometimes Off Topic mentions are appropriate in a message board, but netiquette requests that such topics have OT in the header (so folks can choose to read it if they want to). Folks touting some other, completely non-related stock on this board, for example, would be "spamming" us...unless they were engaging in "chat" with a fellow poster...in which case it would be polite and appropriate to write the header like..."sunfundasst: OT - MSFT"
BTW (by the way) Yahoo seems to be trying out something new, maybe a "spam bot" (?) since many of these "spam websites" mass post the same message to many boards. Noticed that "missing messages" are cropping up on many boards, sometimes within just a few minutes from being posted. Maybe Yahoo is developing an automated way of nipping that junk in the bud. Hope it helps ~Geo