What is the difference between Mental ray & v-ray?

I will politely disagree with Saif Ullah Shah. The realism can be achieved through any renderer, it really comes down the the person's expertise.The major difference is that Mental ray use more like scientific terms and Vray use more understandable terms. In Addition both Vray has further simplified the rendering process by hiding advance setting (which can be toggled with single button). Thus giving user more user friendly approach.Vray is sold separately which is adds up the cost but mental ray is built into Autodesk3D Studio Max and Autodesk Maya. 

Both are the render engines but Vray provides the much realistic output than Mental ray. M.ray is come along with software but Vray is a Plugin thats the most basic difference i can share with you. Thanks