Electromagnetic waves like radio waves or microwaves, are created by the acceleration of electrical charges. They propagate in the framework of space and time. Gravitational waves in contrast are moving gravitational fields, ripples in the curvature of spacetime, generated by the motion of massive objects.
Gravitational waves create a time-varying strain or stretching of space-time, which appears as a fractional change in the distance between two objects. The distance between two spacecraft flying freely in space will oscillate by a tiny amount as a gravitational wave passes through.
LISA will observe a passing gravitational wave directly by measuring the tiny changes of distance between freely falling proof masses inside spacecraft with its high precision measurement system. Key features of LISA are interferometric measurement of distances, long baselines of 1 x 106 km, drag free spacecraft based on inertial sensors, and the familiar “cartwheel”-orbits.