There are two main types of oyster: natives, which are flat and only available during autumn and winter; and rock oysters, which are deeper, slightly less fine for eating raw, and notionally at least available year round.
Oyster beds are monitored for pollution, but the odd rogue oyster will still appear. One Christmas I treated the family to 100 superb-looking native oysters from one of the most respected sources in Ireland, only to have the holiday ruined for the unlucky member who downed a bad one. Always smell an oyster before popping it in your mouth: it should have a delicate aroma of the sea and nothing more.