This belongs in the Site Discussion Forum... now moving...
Spam is essentially posting information that is irrelevant to the topic, or does not elaborate well. For the most part, if you are saying that you agree with something, or are stating your opinion, it is best to explain why. Otherwise, it doesn't contribute to the topic well.
Threads can be considered spam as well. If the thread is too vague / general, it can be considered spam. This is especially true if you try to reply to such a thread, and are unable to create much discussion. Here are some examples. These are not examples of threads that I read before, if someone made a thread like these already, it is coincidental.
"What was the last holo that you pulled?"
"How many Lucky Eggs do you have?"
"How far are you in Ruby/Sapphire?"
These topics ultimately degenerate into merely posting information, as opposed to actual discussion.
If you aren't sure if something that you are going to post is going to be spam, you can always try PMing someone that you know, or a mod, and they should be able to help you.