What is the difference between an air hostess and the cabin crew?

There isn't a difference between an air hostess and the cabin crew.  In actual fact, the cabin crew simply consists of all members of staff on board, and therefore, an air hostess is a member of the cabin crew.

The term air hostess is also not used very often these days, and all members of the team in that role are known as flight attendants.  Here's a brief overview of the different roles of cabin crew members:

• In-Flight Service Manager - also knows as a Purser, the member of the team is responsible for the rest of the cabin crew.
• Flight Attendant - The most well known member of the cabin crew, it is the role of the flight attendant to help passengers in any way they can.  This includes providing health and safety advice, as well as providing in-flight services.
• Medic - the medic is responsible for any health concerns on a flight.  This position is very prominent within air ambulances.
• Loadmaster - the role of the loadmaster is to make sure all passengers and cargo find their way onto the flight safely.

So there you have it.  An air hostess, or flight attendant is a member of the cabin crew, with a specific role to undertake.