There are two main types of vests or body armour available for purchase out there.
1. Ballistics Vest (Fragmentation, Rounds etc etc)
2. Stab/Puncture Resistant Vest (Knife, needle, etc etc)
Depending on your needs and situation you may want to look at different types of vests. For example Mil/LE utilize Ballistic Vests as this is their main threat most often then not. However, even in this category there are main differences between the different levels of stopping power or resistancey to certain projectiles. One standard currently being employed is the NIJ levels.
Often many LE vests would have trauma plate inserts which lessens the effects of trauma on the human body or they can have stab resistant plates inserted as well. Other plates are available with different standards rated by NIJ. Common plates are frag plates, rifle plates or other varying things.
Now stab/puncture resistant vests are what most Ambulance service or bouncer or correctional officers utilize as their likely hood of being stabbed via knives, blades and other pointy objects are higher. The stab resistant vest is just that, resistant, it does not guarantee that nothing will penetrate.
For more info search google for NIJ body armour standards.