There are two types of chestnuts - sweet and edible. Sweet chestnuts have a thin, shiny brown skin with a white flesh. They're the type you roast and eat at Christmas time. Edible chestnuts have a thicker, duller brown skin and a slightly pinkish flesh. They're the ones you use in recipes like stuffing or soup.
To tell the difference between the two, it's all in the skin. Sweet chestnuts have a thinner, shinier skin that's easy to peel off. Edible chestnuts have a thicker, duller skin that's harder to peel off. Once you've peeled them, take a look at the flesh. Sweet chestnuts have white flesh, while edible chestnuts have slightly pinkish flesh.
When it comes to cooking, both types of chestnut can be used interchangeably. So if you're not sure which one you've got, don't worry - they'll both taste great in your Christmas pudding!