Body armor can be classified as soft and hard, according to the material each is made from; on Plate Carrier Zone you can find out more about the differences between the two types. Generally, though, police officers wear soft armor, while soldiers typically use SAPI plates.
Soft body armors are lighter and flexible, and can be worn under a uniform shirt without being visible. Soft armor vests are made from Kevlar threads or Spectra Shield, and are great at stopping slower bullets like pistol and shotgun rounds; however, they cannot protect against centerfire rifle rounds. Level IIA soft armor provides the least protection against bullets, while Levels II and IIIA ensure the most protection.
Note that soft armor in particular is easily damaged by water, heat and ultraviolet light, so make sure to store it properly when you don’t use it, and invest in a quality carrier that is also moisture resistant.
Hard armor is typically made from steel, ceramic, polyethylene plastic (poly plates), or a solid composite (SAPI plates); any of these hard plates can stop rifle rounds. Hard armor is classified as Level III and Level IV protection-wise.
One particular note on ceramic plates: they get cracked quite easily when they are treated roughly and won’t be able to stop rounds as effectively if that happens. Moreover, ceramic plates stop the bullet by breaking up on impact, but that particular part of the plate becomes useless at preventing further bullets from reaching the body.
Heavy and hot
Even the most lightweight body armor is heavy, and is bound to feel even more so once you get moving. Hard armor will slow you down, not only while moving, but also while shooting. Both soft and hard armor will hinder your shooting stance, which means you will need a lot of practice with pistols and long guns.
It will also get very hot, so make sure to stay hydrated. Moreover, because body armor shouldn’t be loose, but fit your body well, you will find breathing more difficult, especially when moving about; the key is to take shallower breaths at shorter intervals.
Armor often comes with a steel trauma plate; it is optional on soft armor, but you must wear it with your hard armor. The problem with this plate is that it makes it easy for a skilled enemy to choke you on your own armor, so bear that in mind.