One of the main (and certainly very impressive) instruments available to particle physics.
Generally speaking, it consists of a big can strung with lots of wires on the inside and filled with a delicately balanced mixture of various gases.
Less generally speaking, these wires are arranged in a pattern that defines so-called 'drift cells': a central, extremely thin, positively charged anode wire, surrounded by several negatively charged cathode wires, usually at a few kiloVolts. If an ionizing particle enters the gas of a drift cell, it will very likely ionize a few of the gas molecules there. The positively charged ions then drift to the cathode wires and aren't interesting any more. The negatively charged electrons drift to the anode wires and are doing a lot of interesting things there, resulting in a wave of secondary ionisation (avalanche) which finally leads to an electronically detectable signal being induced in the anode wire. After that, some expensive electronis converts these signals into digital data and some extremly powerful computers, after a lot of work, finally tell you that you might have discovered something interesting.
A Geiger counter can be thought of as the little brother of a drift chamber. The internal mechanisms are similar, although unlike a Geiger counter, a drift chamber allows (actually, is build to do so) to distinguish specific characteristics of the original particle.