A flame may have elements of a normal message, but is distinguished by its intent. A flame is never intended to be constructive, to further or clarify a discussion, or to persuade other people. The motive for flaming is never dialectic, but rather social or psychological. The flamer wishes to assert his authority, or establish a position of superiority. Occasionally, the flamer merely wishes to upset and offend other members of the forum, in which case he is a troll.
Similary, a normal, non-flame message may have elements of a flame - it may be hostile, for example - but it is not a flame if it is seriously intended to advance the discussion.
A sequence of flame messages is known as a Flame war.
2. "Flaming" is also an adjective used to refer to someone conspicuously and openly gay, usually conforming to various expected stereotypes.