If you're looking for a mask to help battle Covid-19, most likely you've been checking out these three respirator types; N95,FFP2 and KN95. Different countries have different mask classification categories, but these are generally considered the big three.

N95 -this is the USA standard based on NIOSH-42CFR84

FFP2 -Europe; standard EN 149-2001 

KN95 - China; standard GB 2626-2019,

These masks types are all quite similar and they are all tested for for filtering non-oil-based particles such as those resulting from wildfires, PM 2.5 air pollution, volcanic eruptions, or bioaerosols (e.g. viruses). Note that for virus prevention, it is generally agreed that masks with valves should be avoided.

FFP stands for “filtering face piece”
The N in N95 stands for “not resistant to oil”

Comparisons of the three can get kind of wonky, so let me just give a simplified chart showing the general requirements for each of some of the more relevant criteria
:

Filter performance – measurement of the reduction in amounts of specific aerosols in the air that manage to pass through the filter.
N95 ≥ 95%
FFP2 ≥ 94%
KN95 ≥ 95%

Breathing resistance; aka pressure drop: the resistance encountered by air as it moves through the respirator filter. These numbers are slightly misleading, as different countries calculate the resistance based on different flow rates. All three types are actually quite similar.
N95 ≤ 343 Pa
FFP2 ≤ 240 Pa
KN95 ≤ 350 Pa

Total inward leakage (TIL) – the amount of a specific aerosol that enters the respirator being tested facepiece via filter penetration and also face seal leakage. This test is done while the wearer performs a series of exercises in a test chamber. *note that available N95 stats for leakage are based on only a few studies, not a standard NIOSH requirement, so this data is more difficult to compare accurately. As a respirator's performance is largely dependent on its fit to an individual user, there is no standardized test for TIL in the NIOSH approval requirements.
N95 NA
FFP2 ≤ 8% leakage
KN95 ≤ 8% leakage

Strap type - Behind the head elastic straps pull the respirator more tightly onto the face creating a better seal. "Ear savers" are often used with respirators that have ear loops to improve the seal. Ear savers are just a piece of plastic that the loops hook to so the respirator can be fastened behind the head. Also, they have the advantage of being adjustable; ie the user can set the seal to a comfortable level.
N95 behind the head elastic straps
FFP2 elastic straps, or earloops
KN95 elastic earloops

The take-away:

So really, the three respirator types are all very similar. However, during the Covid-19 crisis, there have also been a lot of fake masks on the world market. When purchasing a mask it is good to not only check the mask type, but also check the sellers credentials, and ask to see the mask test report, or certification, as applicable.

Sources:
cdc.gov
esdproducts.eu
academic.oup.com
safetyequipment.org