Yet is used to say that something which is expected has not happened but we think that it will.
The postman hasn't come yet.
In a question yet is used to ask if something that is expected has happened.
Has the postman come yet?
Already is used to say that something has happened earlier than expected.
When is the postman coming? He's already been.
Already is used at the end of a sentence for emphasis.
Has the postman been already?