I don't think the helmet signifies anything, but the smiley is generally dropped into a post to indicate that the poster really enjoyed reading your opening post or reply, and strongly approves. At least, that's what I use the smileys for.
If you go to the reply box and click the smiley on the top towards the right, it lists all the available emoticons. I haven't decided yet what all of them are meant for (some are confusing), but there's a wide array to choose from.
Heck, I forget to use them most of the time...(embarrassed)...
Eric