When buying as Joint Tenants, each party owns the whole of the property; think of it like a pie - each person owns all of the pie. If one party should die, then the whole of the property (so the whole of the pie) is automatically transferred to the survivor.
Tenants in Common
If buying as Tenants in Common, the rules are slightly different. When purchasing a property together in this way, each party has a separate share in the property. If we go back to the pie, Tenants in Common might own the pie 60/40. In this case, if one party should die, then the survivor will not automatically be entitled to the other’s share in the property; specific instructions will need to be left as to how the deceased wishes their share of the property to be dealt with, and therefore it is essential that both parties make a will.
If wishing to hold the property as anything other than 50/50, it is advisable that you obtain a Declaration of Trust which will set out the terms of your purchase together, the shares in which you hold the property and your entitlement to any sale proceeds should the property be sold in the future.