reflections

To achieve realism in 3D renderings, it is very important to understand how reflections work when creating materials. Proper use of reflections also help with creating realistic lighting. Below are some renderings using v-ray in 3DS Max that will help explain the importance of reflections.

 
In this rendering, the material on these utah pots is a diffuse material with a matte finish. There are obviously no reflections.
In this rendering it is the same material with 50% reflectivity. This means the material is reflecting half of the surrounding environment.
This is with 100% reflectivity. This means the object is reflecting the environment without any loss. This is why the large pot gets lost in the sky because it is reflecting the same color. Hardly realistic. The 50% reflectivity above is more realistic than this rendering on the right.

I added a twist to add to realism. In real life reflective objects tend to reflect more where the object is tangent to the viewer/camera. So to achieve this I added a falloff map to my reflectivity.

In this instance where the object is perpendicular to the viewer/camera there is 0% reflectivity, and where the object is tangent to the camera, it is 100% reflective.

HereI played with the perpendicular/parallel falloff. Now where the object is perpendicular to the viewer it has 20% reflectivity, where above there was not any reflection in the center of the objects. This makes it more realisitc.

Here, I continued to play with my falloff map for the reflectivity. Instead of using an even falloff (from perpendicular to parallel/tangent) I used a fresnel falloff. Instead of being even, it has an exponential falloff.

So most of the object is 0% reflective and jumps to 100% reflectivity very quickly around the edges. This is much more true to real-life, and gives good highlights around the edges.

This is the same fresnel falloff but instead of the perpendicular reflection being 0%, I upped it to 20%. This is a good place for our reflections to be.

Another property of reflective materials is how clear they reflect or their glossiness. The next few images demonstrate different glossiness on a 50% reflective material.

In this example the glossiness is 100%. This means that the reflection is accurate and not blurred.

This material is now 90% glossy and the reflections are not as crisp. The reflections are broken up and scattered over the object. This adds to the materials imperfections...adding more realism.
Here the matarial is now 80% glossy with the same amount of reflectiveness as above. As the glossiness becomes less and less it implys that our material is more of a brushed metal.
70% reflective

This is now at 50% glossiness with the same amount of reflection. The reflections in the object are impossible to distinguish because of the amount of scattering.

Understandint these properties in materials is essential for creating realistic scenes in 3D. Using these material principles with a combination of good lighting will give you a good head start on creating realistic renderings.