As far as I know;
Logo: graphic symbol which represents a company or organization.
Visual Example:Microsoft Windows Logo
Generally, Logos > Avatars (Not always)
Avatar: (Internet) online image that represents a user in chat rooms or in a virtual "space".
Ex: Your msn messenger display picture can be your avatar. or can be something like below.
Icons: small graphic symbol which symbolizes a program or file in a graphical user interface (Computers); or there are emoticons whic describes the users feeling visiully.
And symbol is a more general term,
I am neither a NES nor a Teacher , if I am wrong please correct me
Student or Learner