A necro-post is a new reply posted to an old inactive topic. The word necro means corpse and as such the act of necro-posting is replying to "dead" topics
Necro simply means you posted in a very old thread. The difference between this and reviving is the fact that necro'ing did nothing to contribute to the thread. Your post is (likely) forgiven if your post contributes to the thread, meaning the post actually has something to do with the thread and helps generate more discussion.
"Lol! A very old thread!" is a necro, whereas "I think X is cool because Y, Y, and Z. I also think that Q and F are cool too, because..." is a revive.
[dup] is duplicate thread, [ans] means the thread was answered, [req] means the OP wished for the thread to be locked, and [READ THE STICKIES!] means read the stickies.
I have never seen [req] or [READ THE STICKIES!], so those do not contribute to the answer. However, thank you all for the answer to [ans], [dup], and [necro].
