well both domain and workgroup are a collection of user. Domain requires u to be explicitely set part of it by the admin via user name/passwd and workgroup is just a group of computers without any security.. ie anyone can login and use the workgroup's resources. where as in a domian a user can be assigned what he can do and what he cannot.
basically a domian is a workgroup with enforced security.
A DNS (domain name server) is a server that tell your browser about the IP address that is associated to a website. Comps dont understand
www.yahoo.com but a ip address so when a user types in a web adress the DNS tell it what the ip address associated with that url is.
do a google u will find loads of info