Gravitational lensing is the focusing of electromagnetic radiation by a massive body. EMR such as visible light passing near the massive body (such as a galaxy or galaxy cluster) is turned from its straight path by the force of gravity. The setup is called a gravitation lens. A galaxy has the mass to lens a more distant galaxy or quasar.
Gravitational microlensing is the gravitational effect on electromagnetic radiation by a smaller body such as a star, and is a means of detecting the presence of bodies otherwise unseen but able to lens the passing light enough to create a transient. The smaller body can be a transiting planet, dim binary star companion, brown dwarf, or "free" planet. Searches for such lensing transients have been carried out to search for MACHOs.
Gravitational lensing of a jet can be useful in observing jet and active galactic nucleus detail otherwise not visible. A jet which ought to be straight and appears not to be offers one means of noticing such lensing.