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)...
I use them alot and I thnk that they are just to lighten things up maybe. I like the dude with sunglass cause I'm in the sunshine state. Other then that we have a desent amount of them but I thought Abigail was gonna work on Jeff to get us some really good ones. I will but it on my Christmas Wish List.