After a short review of the properties of gravitational waves as described in Einstein general relativity, the problem of characterizing suitable detectors is addressed in order to define suitable parameters for comparison. The concept of sensitivity as a combination of frequency response function and noise budget at fixed signal to noise ratio is used, looking at the performances of existing optical gravitational waves antennas and of a symmetric Ramsey-Borde (Mach-Zehnder geometry) atom interferometer studied as a prototype of new detectors for gravitational radiation.