Can someone explain the "faces" to me?

I'm trying to figure out the meaning of the face symbols to the left of the thread title.

Obvioulsy I can decipher, happy, sad, angry, questions, etc. But after reading some of the threads I don't understand why some get tagged angry, unhappy etc when I don't see evidence of that type of conversation "in" the thread, so who decides the tag and who put it there?

Thanks

 

The one who posts gets to choose what smilies and symbols he includes in the post.
At the bottom of this page are "posting rules". There are alot of things you can do to your posts as per your choosing.

 

I see, you can actually put the face in the "title" of the thread as well as the text...