Source: http://www.uptodate.com/contents/image?imageKey=DERM%2F69573&topicKey=DERM%2F13648&source (1) Yadav, RK. (2009). Definitions in Laser Technology. J Cutan Aesthet Surg. 2(1): 45–46
IPL (or Intense Pulsed Light) and Laser (or Light Amplification by Stimulated Emission of Radiation) are two forms of light therapy commonly used in the aesthetic industry for purposes of permanent hair reduction, vascular treatments, skin resurfacing and rejuvenation depending on the type of light used (IPL/Laser) for the appropriate skin color. IPL is not only used for hair removal but a wide variety of skin pigmentation, vascular reductions and skin rejuvenation treatments due to its broad wavelength that can target several pigments in the skin. Both treatment principles involve targeting chromophores (color bearing light absorbing particles) like melanin in the skin or hair follicle and haemoglobin in blood vessels.
Of the entire range of light, or what is called an electromagnetic spectrum, light can either be produced in the visible range which is used in IPL or in the form of a monochromatic invisible light which is called a laser to selectively target chromophores such as melanin in the skin that simply absorb this light producing heat. This is called selective photo-thermolysis. Both processes (Laser/IPL) gradually heat up the unwanted hair/pigment in the skin over time, eventually diminishing its growth giving you a permanent hair or pigment reduction.