The Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation, WHISPER, performs high frequency electric field measurements on the four satellites of the CLUSTER mission. In active mode, the WHISPER behaves like a classical topside sounder. It provides, via the identification of resonances that are excited at characteristic frequencies of the encountered plasmas reliable and accurate determination of the total electron density and magnetic field strength. Whenever the transmitter is switched off, the WHISPER becomes a simple wave receiver (passive mode). The 2 to 80 kHz frequency range of the electric component of natural waves is then monitored. The main objective of the presentation is to highlight the plasma and natural waves diagnosis capabilities of the WHISPER instrument. The way the plasma resonances are extracted and identified is pointed out and results obtained from the solar wind down to the Earth's plasmasphere are shown.