Lights
No matter how much care you put into realisation of models and textures, a bad lightning will cause a 3D picture to look really ugly. Lightning is one of the most important things to set up correctly to achieve realistic results.
There are several kind of lights:
Point light
Spot light
Ambient light
Area light
Volume light
This is the simplest light and consist of a infinite small light source that emits light in every directions. rendering algorithms that calculates real shadows will cast sharp shadows that are not extremely realistic.
Spot lights cast a cone of light so you have to set up also the direction of the illumination. Usually 3D software that support spot lights allows the definition of the "falloff" parameter that cause the circular spot of light casted on objects to fade softly.
Note that this does not mean that objects cast soft shadows. This again depends of the shadowing algorithm used.
Ambient light is an overall light that gives a basical illumination.
In raytracing there is an "ambient" texture parameter that simulate this light. Some software provides a real ambient light factor that is usually a infinitely distant light that cast parallel light rays, like the sun (ok, sun is not infinitely distant but it rays fall on the heart nearly parallel).
To avoid sharp shadows one solution that works even in raytracing is to use light source that has an area. POV-Ray fakes area lights by using an array of pointlights. It works but it multiplicate by n the rendering time (where n is the numbers of pointlight used to make the area light).
This light source is similar to area light but it has a finite volume. Some software ,as TrueSpace for example, allows to use every object as a light source by letting it emit light. This is not a common feature.
For all the light sources yet seen it's possible to set up the amount of luminosity and the color of the light emitted.
Some software allows to use a bitmap to project an image or a particular shaped shadow, ("Gels" in StudioPro), to create effects like light rays casted through a closed window...
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