Joint tenants have a common undivided interest in the entire property; that is, everyone owns the whole thing in common with everyone else. This means, in practice, that the last person left alive gets the whole property, because there are survivorship rights.
Tenancy in common means that each owner owns a percentage share, whether equal or not, depending on the decision at the time they took title. On death, each owner’s share passes to their estate.
Whether you own as joint tenants or tenants in common depends on several factors. If you are buying with your spouse, out of joint family funds, then you may be more inclined to joint tenancy. If you are buying with a business partner or friend, or with a partner you are about to move in with for the first time and are investing unequal shares, you may be more inclined to tenancy in common.
You should also note that, at any time, a joint tenant can sever the joint tenancy and make their share in common – and can do this with no notice to the other owner(s).
Before you buy, you should have a conversation with your lawyer about the best way for you to take title.