In photography, light is in fact the single most important aspect of consideration.
What time of day in which you choose to shoot can affect both how the subject will be represented, as well as your technical limitations in presenting it. When photographing it is wise to make use of the 'slowest' or smallest ISO film/medium possible... This reduces 'noise' in the case of digital, and grain in the case of film. Of course, there are exceptions to this, when the surreal/aged feel grain or noise adds to a photo is desirable, however for the most part, the high picture quality granted by slower sensitivities is preferable.
What separates a professional looking photograph from an amateur snapshot is both the quality of the light, and the quality of consideration placed on 'how I will deal with this type of light'.
For instance, if one was photographing a scene in which motion would be better represented slowly, a sudden need for more light arises. More so if coupled with that is a need for great depth of field. If at all possible, a photographer must make considerations such as; time of day (when will the light be strongest) and weather (does the skycast offer greater or lesser contrast in the photograph).
Most of these requirements are greatly reduced in difficulty when shooting in a studio, as the light is directly controlable, but a new difficulty arises... One must aspire to create a light that simulates light that humans have over eons come accustomed to seeing... One must simulate 'nice' light.... One must aspire to simulate the ways in which the sunlight shines on the subject.
The direction of the light is a critical consideration;
For instance, in portraiture, one would rarely use a ligh source that casts shadows upwards onto a face... That is to say, one would generally avoid placing the light below the subject, otherwise fall victime to 'scary campfire light' as is created by underlight flashlights to scare children... this light is both un-flattering, and un-natural.
Instead, to create a quality photograph, photographers try to emulate the suns' representation of light... That is, a light that falls both on the subject (from above) and on surroundings, and then reflects back onto the subject. This reflecting property of light is key. By placing a reflective surface opposite the shadows on a subject, one can fill the darker areas with a 'soft' or diffused type of light. The difference from such a subtle chance can be astounding.
Contrast is the measure of the difference between the shadow and highlight brightness in a photograph.
Low: A low contrast photograph, having a lower contrast grade will look more greyish in the case of black and white, and will be subject to a muted feel in color.
Hard: A hard contrast image will have both very bright highlights and dark shadows, and the lines which define the shadowed areas will be much more significant and remarkable.
A between might be considered when shooting... A contrast grade that neither enhances or reduces dramatic effect in a photograph might be called 'normal' contrast or 'medium' with the advent of digital.
while contrast can be controlled in the darkroom/digital darkroom, it is wise to make considerations when shooting... What sort of contrast do you want to emphasize should relflect the nature of the light your shooting in.
Color temperature (Kelvin)
Hard light; A light which casts heavy, clear, profound shadows when interrupted by an opaque object. Hard light gives high contrast and produces vivid and vibrant colors.It is generally used to increase drama in the picture.
Soft light; A light whose shadows are cast in a more soft, and subtle way. Softer diffused light can be created by increasing the size of the source with respect to the subject which can further be diffused by using filters (translucent materials) or by reflecting the light off an intermediary surface (generally walls-thereby further increasing the size of the source) before it falls on subject.