Thread (with a big "T") is a
java class ; see java.lang.Thread. When people say "a thread" they usually mean "an instance of class thread" or "an object of type thread".
join() will cause the current thread to stop, and wait for the thread on which it is called to finish, before continuing.
Only non-private members are inherited by a sub-class. Constructors are a special kind of member, they are not inherited. Neither are private methods or variables. Everything else is inherited.