A piezoelectric element generates a charge when stressed.  Charge mode sensors simply output this generated charge and require an external amplifier to convert from charge to a usable voltage.

PCB pioneered the ICP concept which places a charge to voltage converter inside the sensor to give a low impedance voltage output directly from the sensor.

Charge mode sensors have the disadvantage that they need expensive external amplifiers and higher cost low noise cabling.  They suffer from tribo-electric noise if the cables are not securely fixed down.  However, they can withstand much higher temperatures than ICP mode and can work up to 1200 Fahrenheit (650 Centigrade).

ICP mode sensors have a direct low impedance voltage output and need a low cost constanct current power supply to drive them.  They can work with standard co-axial cables and do not suffer from interference in the same way as charge.  However, typically they are limited to 250 Fahrenheit (121 Centigrade) with some high temperature versions working up to 325 Fahrenheit (163 Centigrade)