People call Photochromic lenses by all different kinds of names, However, they are all Photo chromic lenses.
Photochromic lenses are clear lenses that react with UV and produce a tint effect based on the amount of UV present.
Transitions is actually a brand name of a Photocromic Lens. Although transitions are probably the most advanced photochromic lens on the market there are many others out there.
When wearing Photochromic lenses even a dull day some reaction will occur producing a very pale tint and then on a very bright day where more UV is present they will turn quite dark. When wearing them indoors they will become clear.
The idea behind photochromic lenses is that they will provide the correct amount of protection for the varying light conditions you experience. However, when driving, your car windscreen will block a certain amount of UV which is required to make the lens react so they will not work as well as they would outside of the car.
Photocromic lenses are ideal in the sense that they can save you having to purchase two pairs of glasses ie, Clear glasses and a pair of Sunglasses.