Each tenant in common has a separate interest in the property, which he can dispose of as he chooses. If B dies, and in his will gives his interest to D, now C and D are tenants in common. Joint tenancy is just the same, with one big exception. That is, when one dies, the other automatically gets his interest. If A and B are joint tenants, and A dies, B gets A's interest regardless of what A's will says.
Joint tenancy is useful because it keeps the property out of probate, which saves time and money.
To create a joint tenancy, the tenants must acquire their interests at the same time, through the same instrument, and each tenant must have an equal interest. Tenancy by the entirety is like joint tenancy, but is available only to married couples, and cannot be severed unilaterally by one party. If A and B are tenants by the entirety, A cannot sell his interest to C. A and B have to sell together. Tenancy by the entirety is terminated by divorce. It exists in only about half the states.