Electron Multiplying CCD (EMCCD) cameras can be used to detect very weak signals, which would otherwise be lost within the noise floor of the camera. Using a conventional CCD to detect these very weak signals, usually requires both long exposure times (to integrate a detectable amount of signal) and slow readout rates (to minimise the read noise). However, EMCCD cameras can detect these same signals using much shorter exposure times and higher readout rates. This is done by applying EM gain to the signal before it reaches the output amplifier of the CCD sensor, effectively reducing the read noise of the sensor by a factor equal to the amount of gain applied.
Understanding electron multiplying gain