It is made of a durable, thick red rubber. They make sizes from xs to xxl. Sizes overlap, for example, the medium is for dogs 15-35 lbs and the large is 30-65 lbs. If your dog is in between sizes, I would go with the larger size. Kongs come in many durabilities: puppy, regular, extreme and senior. If you have a puppy that is under 6 months then choose the puppy as it is softer and you won't run the risk of chipping puppy's tooth.
You can fill the Kong with peanut butter, yogurt, cheese, bananas, blueberries, apples sans seeds, dog food, etc. Kong even provides lots of "recipes" on their website for filling the Kong. They even have one that is like a five layer dip for dogs. You can even freeze the Kong and it will make the filling last longer and for those teething puppies it eases the pain by numbing the gums.
Personally, I only give my dog her Kong when I leave and put her in her crate. I pull the Kong out of the freezer and she runs to her crate. She loves it!
The Kong is great if you have a dog with separation anxiety. My dog had mild separation anxiety when I got her. Before I got the Kong, I could here her from the driveway crying because I had left and we have a two story home. After the Kong, I didn't hear a peep. Dogs with separation anxiety can benefit from having a toy that they only get when their owner leaves. It's like a symbol for them. For my dog it means, "Ok, mom is leaving but she's coming back and I know this because I have my Kong."
The Kong can also help with those destructive dogs. Most destructive behaviors happen within the first 30 minutes of the owner leaving because they are so focused on their owner leaving and they get themselves worked up. If you give your dog a frozen Kong full of peanut butter then their mind is busy with the Kong and not busy ruining the furniture.
I definitely say that everyone needs to have a Kong in their arsenal of toys. As their slogan says, "Your dog will love you for it."
My suggestion...watch where you step when walking...it's definitely not something pleasant to step on.