IEPE refers to a type of transducer that is packaged with a built-in charge amplifier or voltage amplifier. IEPE is an initialism for Integrated Electronic Piezoelectric.
Integrated Circuit-Piezoelectric, or ICP®, is a trademark of PCB Piezotronics, Inc., and refers specifically to the IEPE devices that they manufacture. Their website contains a good introduction to piezoelectric accelerometers that can be found in the Additional Resources section.
IEPE technology is marketed under several recognizable brand names. Although other companies make IEPE based transducers, this does not imply that they are compatible with equipment that claims ICP® compatibility. One of the key variations from company to company is the amount of current necessary to power the sensor. Generally these sensors require a 4 mA current excitation, but be sure to verify this with the specific device being used.