Towing

Tare Mass - Empty weight of trailer
ATM - Aggregate Trailer Mass - Total Maximum weight of trailer including payload.
GTM - Gross Trailer Mass - ATM minus Towball Mass i.e. the weight on the wheels.
Tow ball Mass - The weight on the tow hitch
GVM - Gross Vehicle Mass - The weight of the vehicle plus tow ball mass.
GCM - Gross Combination Weight - GVM + GTM.

For a great explanation of what makes a caravan stable (and unstable) see Collyn Rivers article.

To measure your towel weight, there are scales available from Towsafe and Hayman Reese.