...un riche marchand dont la fille préférée s'appelait Belle.
...a rich merchant whose favorite daughter was called Belle.
Et grâce à lui, j'ai rencontré beaucoup de gens très intéressants, dont Gilles Proulx.
And thanks to him, I met lots of very interesting people, including Gilles Proulx.
It's usually pretty easy to distinguish these two uses of dont from context, but punctuation also provides a clue: dont is usually preceded by a comma when it means "including," but not when it means "whose."
J'ai un chat. Le poil de mon chat est très doux.
I have a cat. My cat's fur is very soft.
J'ai un chat dont le poil est très doux.
I have a cat whose fur is very soft.
As you can see, dont stands in for de and refers back to chat. It also prevents the redundancy of saying chat twice.
Et puis il y a une chose dont Michel est particulièrement fier.
And then there is one thing that Michel is particularly proud of.
...dans la ville de Dongtan en Chine, dont nous avons déjà parlé.
...in the city of Dongtan in China, about which we've already spoken.
Voici le livre dont j'ai besoin.
Here is the book that I need.
Et puis Michel est particulièrement fier d'une chose.
And then Michel is particularly proud of one thing.
Nous avons déjà parlé de la ville de Dongtan en Chine.
We've already spoken about the city of Dongtan in China.
J'ai besoin de ce livre-ci.
That about covers it for dont! Though the scope of its applications can be a little daunting, it's a very useful and succinct word that will make your French sound very sophisticated. Don't neglect to use dont whenever you can!