In Maya, the samplerInfo node is responsible for a ton of neat shading effects, including realistic glass, velvet, fake rim lighting, and interactive z-depth shaders. All the samplerInfo node does is return properties of the object that is being rendered during render time (such as the world space position of the point of the object currently being rendered). I'm not going to go into the details of all of these properties here; check the Maya documentation for detailed descriptions of any of the attributes I don't mention here.

To quickly see what this thing can do, create a Blinn shader and assign it to a sphere. Then, in the Hypershade, go to Create > General Utilities > Sampler Info, or use the Create tab on the left side of the window. Then create a Ramp texture, and delete the associated 2D or 3D Placement node attached to it. Edit the ramp to make it a V Ramp that ranges from black to white, with white at the top (position 1.0) and black at the bottom (position 0.0).

Now in the work area you should have the Blinn shader, the ramp texture, and the samplerInfo node. First, connect the facingRatio attribute of the samplerInfo node to the vCoord attribute of the ramp texture. If you don't know how to make connections in the Hypershade you shouldn't be reading this tutorial hold down CTRL and middle-mouse drag from one node to the other to open up the Connection Editor. Then connect the outColor attribute of the ramp to the Transparency attribute of the Blinn. Your shading network should look similar to the picture on the left.

When you assign this shader to a sphere and then render it, your object will look pretty boring until you view the alpha channel, which should look something like the picture on the left. Notice how the object is more transparent in the center than on the outside? This is what the facingRatio attribute of samplerInfo is all about.

FacingRatio looks at the surface normal of the point on the object that you are currently rendering and returns a value from 0 to 1 depending on which way the normal is facing relative to the camera. If the normal is facing straight at you, it returns 0. If the normal is facing perpendicular to the camera's line of sight, it returns 1. In other words, facingRatio says "0" if the face of the object is pointed towards you, and "1" if the face is at a glancing angle towards you. What you just did was take that number and send it to the vCoord of a ramp, which coincidentally has colors at positions ranging from 0 to 1. The color information from the ramp at that position is then sent to the transparency attribute of the shader.

This effect is the basis for any glass or water shader: it's more transparent and less reflective when you look at it straight on, and less transparent and more reflective when you look at it at an angle. All you would have to do to make this shader look like glass, other than lighting it properly and including an environment to reflect, is create another ramp, connect it to the samplerInfo node like before, and connect the outColor of the new ramp to the shader's Reflectivity attribute. Then it's just a matter of tweaking the ramps.