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.
Angular Power Spectrum
Cosmic Lens All-Sky Survey (CLASS)
Deep Lens Survey (DLS)