Under a Joint Tenancy, each owner has an indivisible share in the property, meaning that each owner is equally entitled to the whole of the property. The key feature of a Joint Tenancy is the right of survivorship.
The right of survivorship means that on the death of one co-owner, that co-owner’s interest in the property will pass automatically to the surviving co-owner(s). Therefore, if you own property as a Joint Tenant, you cannot leave your share of the property to someone in your Will; it will simply pass to the other Joint Tenant(s).
Through the right of survivorship, the last surviving Joint Tenant will end up owning the entire property. On their death, the property will form part of their estate. As such, they will be able to leave the property to whoever they wish in their Will. The last surviving Joint Tenant will also have all of the usual rights of sole ownership, such as being able to sell the property or give it away etc.
Tenants in Common
With a Tenancy in Common, the co-owners still own the property jointly but each will have a distinct share in it. For example, if you own property as Tenants in Common with one other person, you might each have a 50% share of the property. The shares do not have to be equal in a Tenancy in Common, however. The shares might be in different proportion, such as 60/40, perhaps reflecting how much each of you contributed to the purchase price.
In many cases where a property is owned as Tenants in Common, there will be a separate document indicating the share which each Tenant in Common holds in the property. This is called a Declaration of Trust.