A hollow-point bullet is similar to any other bullet, except that there is a hole in the tip, hence the term "hollow-point". The depth of the cavity generally ranges betwen 3mm and 5mm. The design forces the bullet to expand greatly upon impact.
When the bullet strikes a soft target, the pressure in the pit (hollowed point) forces the ring of lead around it to expand greatly into a mushroom-shape. This causes considerably more soft-tissue damage than had the nose stayed intact. This also causes the bullet to "tumble", creating more damage.
Generally, ideal performance of a hollow point bullet occurs when the bullet expands in a uniform shape, to the full depth of the hollow (or slightly more), and does not fragment. In theory, the deeper the hollow, the greater the expansion of the bullet.
There are two varieties of hollow-point: "Jacketed Hollow Point" (JHP) and "semi-jacketed Hollow Point" (SJHP). The difference is in the metal jacket surrounding the lead bullet. In a JHP, all of the lead is covered, except for the pit. In a SJHP, a small area of lead is exposed leading up to the tip. The amount will depend on the bullet design itself. a Slang term for JHP is also the "dum-dum bullet". The actual dum-dum design is not a true hollow point projectile.
"The Hague Convention prohibits the use of expanding or fragmenting bullets in warfare (often incorrectly believed to be prohibited in the Geneva Conventions), but hollow point bullets are one of the most common types of civilian and police ammunition. In fact, in many jurisdictions it is illegal to hunt game with ammunition that doesn't expand, and many target ranges also forbid full metal jacket ammunition (which is more likely to ricochet)." -Wikipedia