@oristarA,
Ori, post is shorthand for postal service et al. Back in the day
, if you wrote a letter, it was posted with postage stamps down at the post office or post box. Similarly, If you lost a cat, you'd post a message on a telephone post or a board.
Now-a-days you might
send an email, it's not really common vernacular, at least in my circles to post an email, but they may in other locales.
A2K is basically a message board. The messages or questions are posts. The notes within or under each title are also called posts and collectively they make a thread. A thread isn't necessarily a theme as many go off topic and become tangled and die. Some eventually get back on course and go on and on and on and you get the picture. And others never really get a start. Just like a piece of thread.
The act of writing a question, starting a conversation or replying to anything is considered posting. Once you're done posting and you've posted or replied to something in a thread, it's past its prime and it from then on it becomes a post.