yes, they are expressions.
:]. :) means something like "I'm glad" or "I'm happy"
:p - I am not sure, but I think we use that to say that we understood something.
:D or :d - means something funny
:(, :/ , :[- unhappiness
:s - that means we don't know what to say or we are sorry for something.
If I don't write something, tell me about that and I'll try to translate.