Think of something hard breaking in a crisp, snapping manner, and you've just imagined a fracture. The word is most often applied to a broken bone, but it can used to describe any sharp, sudden break of something solid.
The Latin frāctus means "broken," and its descendant fracture can mean any break, though it's most often associated with a hard — maybe even brittle — material, such as a bone, a rock, or the earth’s crust. When something softer is split we say it is torn. For example, when we say someone broke an arm, we are referring to the bone, not the muscle; we'd say the muscle is torn. When someone funny "breaks us up," we might say "you fracture me!"