grant public to a user

I have not really worked with synonyms before. Some of the synonyms in EBS system belongs to PUBLIC group. If I grant a custom user with the grant PUBLIC to customuser, what kind of privilege does this user going to have:? I don't want the customuser to be able to change any base table data. Will this PUBLIC privilege give the customuser insert, update, and delete privilege to any of the oracle base tables? Please advise. Thank you very much.