Your avatar is a photo of your dog, and your profile picture is a photo of you and your dog.
Avatars are the pictures that show next to your posts, and profile pictures only show on your profile.
If both photos showed next to each post, it would look too cluttered and distracting.
Yeah. I don't know how common it is on forums for there to be both an avatar and a profile picture but on most forums, the avatar is what shows up on your posts (if you have chosen or uploaded an image for your avatar, that is). I can't really remember the last time I had a profile photo here.