What is Joint Tenancy?

If you mean ‘joint tenancy with rights of survivorship’ and you are referring to real property, then both parties own the property fully, subject only to the rights of the other party.   When one of them dies, the property fully belongs to the other.  This is a useful estate planning tool and is superior to a ‘last will and testiment’ for this purpose.  Upon the death of one of the joint tenants, all you have to do [in most places] is record the death certificate and title is vested in the survivor. 

This is in contrast to ‘tenants in common’, where the death of on party results in the substitution of his heirs in his place and stead.  See a local lawyer that does real property law, as some states have community property or tenancy by the entireties as well.