This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
Basically, a spoiler is any piece of information about an
episode that would spoil a part of that episode for someone
who hasn't seen it yet. Spoilers can be gleaned from watching
commercials, reading TV Guide, rumors, or what have you. Some
people like to know this stuff in advance, but a lot of other
people like to go into an episode knowing nothing.
At the start of the third season, ABC showed in their
commercials for the season premiere that one of the characters
would get shot. Not everybody on the group had seen the
commercial, and one person started a thread entitled
"(Character's name) gets shot!" which basically spoiled the
surprise for everybody else. The same thing happened near the
end of the third season, when several people found out from
magazine articles that one of the recurring characters was
going to be killed off, and put that information into the
heading of their posts.
Because some people don't mind knowing stuff in advance, it's
okay to include spoilers in your posts, but there are a few
basic guidelines to follow to avoid ticking people off:
1) Always insert the word "Spoiler" in your subject header,
especially if you're following up to a thread that didn't
originally have spoilers in it
2) NEVER put an actual spoiler ("I can't believe Medavoy is
going to go to jail!", to make up an example) in your
subject header. Even if you were to leave Medavoy's name
out in that particular example, the mere knowledge that
*any* of the characters is going to jail constitutes a
spoiler for most people. A good example of a spoiler-safe
header is something like "What I heard about the season
premiere - Spoiler." And if somebody else inadvertently
includes a spoiler in his/her header, and you want to
follow up to that post, please delete the spoiler info
from the subject line.
3) Try very hard to include a lot of blank lines in a post
before the spoiler information, so that some unfortunate
soul whose newsreader won't allow him/her to scan subject
headers, won't unwittingly stumble across a spoiler.
So, you may be asking, do we have to do this for every episode
in perpetuity? No. The generally accepted Usenet rule is that
once an episode airs, spoilers don't apply to it.