This is a picture of all 4 different darts. From left to right, they are: Small Snap Cap, Large Snap Cap, Light Sili Tip, Heavy Sili Tip. I believe they are also suited to blaster of proportionate power - weaker blasters on the left, the most powerful on the right.
Snap caps are small pieces of plastic shaped similar to a dish, such that it can hold a weight inside it. Should the snap cap break off, the weight breaks off with it. They are quite hard.
The sili tips are tips made of silicone (ORLY). They have been designed specifically to provide the optimum weight and shape for Nerf blasters.
In my snap cap darts, I've pulled off the tip of a streamline dart and used that as a weight. As a result, it provides an ideal weight for low power blasters. The two silicone darts are highly accurate and also provided a decent range boost.
In conclusion, I think this is worth your ~$10 - about $5 of it is shipping.