There are primarily two types of dibblers: those for planting seeds, those for planting bulbs. Bulb dibblers are typically longer with a fatter body and point to create a large hole. They have a flat point to create a firm contact point for the bottom of the bulb, and lines at one-inch intervals for reference.
Seed dibblers on the other hand, are typically skinny and pointy, to create a small hole for starting seeds. Some dibblers will have half-inch marks, and then inch increments.
You can also use the one-inch marking to ensure you place your seeds/plants the correct distance from each other.