HEPA stands for "High Efficiency Particulate Air".
It's a filter able to trap small particles typically missed by vacuuming. These small particles don't get recirculated back into the room when they are caught by an authentic HEPA filter. This is a great boon for allergy sufferers and others with breathing and health challenges.
A genuine HEPA filter must perform at a pre-determined standard. It will have a serial number and it must trap at least 99.97% of particles which are .3 microns. This information will be printed on the filter, assuring you that you have an authentic HEPA filter.
Some companies offer "HEPA-like" filters that resemble the real, high-standard-meeting filter, but in fact do not meet the 99.97% standard. They are usually less expensive; but they don't perform as well. That's an important distinction for allergy sufferers who need clean air.
HEPA Corporation uses authentic HEPA filters. Always.