One of my biggest pet peeves is when people call turkey vultures "buzzards." (Buzzards live in Africa, not the United States.) Another pet peeve of mine is a similar misnomer: when people refer to opossums as "possums." They're not the same thing!
If you live anywhere in the Western Hemisphere, you have never seen a possum in the wild. You have likely, however, encountered an opossum or two. The term "opossum" refers to any of 103 species of marsupial living in the Western Hemisphere, including the notorious, rat-like Virginia opossum. Virginia opossums are often colloquially referred to as possums, but this is incorrect.
True possums live only in Australia (and nearby islands). Like opossums, they are marsupials, but they are not closely related. There are about 70 species of possum, including the brushtail possum and the nectar possum. Scientifically, possums are opossums are classified as very different animals-- sharing the marsupial pouch trait and similar names, but little else.
In other words, if you're in Australia, you've seen possums. If you're in North America, you've seen opossums. They're not the same thing at all!