Smileys are small graphics that have the purpose of expressing feelings and thus underlining the written opinion. If you chat every now and then, you are probably familiar with smileys. Some "standard strings" are automatically converted into smileys. Just tilt your head to the side and with a little imagination you should be able to recognize a face.
If you don't want to use the graphic smileys in a post, you can use the 'Disable smileys' option when composing a post. Then it will be ;) not converted into a graphic smiley, but remains as ;) stand in the text!