I know what bunions look like, but what exactly are they?

Misalignment of bone. Bunion are structural deformities that are progressive. They are, normally, secondary to abnormal foot function. The foot rolls in too much causing the big toe to be pushed towards the second toe. The big toe then pushes the first metatarsal bone the opposite direction. Over time this leads to the bony prominence "bunion". The joint becomes misaligned over time and becomes symptomtic. Dr l.
Enlargement of bone. In a bunion deformity the first metatarsal adducts or moves away from the midline and the great toe abducts or moves toward the midline of the foot. The excessive motion in the metatarsal bone causes enlargement of the metatarsal head causing a bunion deformity.
Abnormal "bump" A bunion is a progressive disorder that is often described as a bump on the side of the big toe joint which may become painful. The visible bump actually reflects changes in the framework of that joint and possibly adjacent joints. In later stages, the big toe may lean towards the second toe which throws the bones out of alignment – producing the bunion’s “bump” which may lead to pain.