A bolster adds weight and control, and makes a knife more comfortable to grip. The tang is the continuation of the blade into the handle. There are different sizes and shapes. This gives you the best balance and durability. A handle is either riveted or molded onto the tang.
Handles come in several materials.
I like wood or resin-impregnated wood. They feel secure in my hand because they repel grease. Some good knives have handles made of molded plastics. These handles tended to fit the shape of my hand better. You can't see it, but they have an ample tang called a rat-tail.