Jacketed Hollow Point ("JHP") Bullets
Jacketed Hollow Point, or JHP, cartridges are called "hollow points" because the tip of the bullet looks as if it has been drilled out, leaving a hollow portion in the tip of the bullet. They are typically a lead core encased by copper, hence the "jacketed" part of the name.
Oftentimes, JHPs have lines that run down along the outside edge of the copper jacket. This, combined with the hollow tip, allows the bullet to reliably and consistently expand, or "mushroom" out, causing it to lose velocity faster, impact more surface area, and transfer as much energy to the intended target as possible. The loss of velocity also reduces the chance that the bullet will pass through the target to unintentionally hit something on the other side.
Because of the above qualities, many law enforcement agencies choose to carry JHPs, and JHPs are typically recommened as a great self defense cartridge.