Can you tell me why each time a post a thread that a letter "S" inside a circle appears next to my name ? I'll assume that it's happened on this post, if not check my previous posts to see what I mean. Thanks, and keep up good work !

It means your subscribed to that thread, as in when someone posts in that thread, you will get an email if you clicked that option!
If you click on that S, it should tell you which threads you will be getting mails on.