Spoilers = information about what is going to happen next on the show, based on news reports, leaks from the production, seeing the episodes early, etc.
Speculation = guesses about what is going to happen next on the show, based on nothing reliable - basically based only one's own personal instincts
Generally, "Speculation" is allowed in a "No Spoilers" thread.
Also, generally, discussion about what
has happened on the show (i.e., past tense) is allowed in a "No Spoilers" thread. That means that you can talk about what happened in an episode after it has been broadcast. That last bit is a little sticky, since with many shows, it is broadcast at different times in different places. Generally (I gotta keep saying that because otherwise someone might think I'm saying something absolute

), for shows first broadcast in the US, the convention is that you can start talking about an episode that was broadcast, after the broadcast of the episode takes place
in the Pacific Time Zone. Occasionally, you'll see a thread with an East Coast bias, and folks will start talking about an episode just after (or even while) it is being broadcast on the East Coast. That's bad form, as far as many folks are concerned.
Of course, those parameters assume that the program is a US-based program, and isn't being broadcast abroad. That's increasingly not the case. In some cases, a show is broadcast in the UK a few days before it is broadcast here in the US, or in Canada a few months before it is broadcast here in the US. In such cases, sometimes, you'll see a US bias, and so you cannot discuss the episode until it is broadcast in the US, but generally the principle is that you cannot discuss an episode in a "No Spoilers" thread until it is broadcast in the
westernmost time zone
of the country of first broadcast. Therefore, "No Spoilers" threads for shows like Doctor Who and Durham County may indeed contain spoilers, from the perspective of US viewers.
Of course, these are all just general guidelines followed by "No Spoilers" threads across the Internet. Each thread can, itself, vary from these guidelines as it sees fit (but generally few do so).
Now: Aren't you glad you asked?
