The source of light:
We are surrounded by different sources of light. If you’re indoor, your light source may be fluorescent light, incandescent light, candlelight, a window, or (often) a combination. Outdoor, apart from the sun, your scene may be lit by street lights, fluorescent light or even moonlight. You have to cater for all of the above when you are photographing. Every source have its specific characteristics.
Our eyes have an impressive ability to adjust to the available light conditions, achieving some kind of “normality”. It is not the case with a camera, which will capture the light as it is. This is why every camera has a white balance control that will try to simulate what your eyes do.
The color of light:
Even though natural light is has one light source – the sun – it is the most complex type of light, and by far the one with most color varieties. There are so many factors that affect sunlight (clouds, haze, temperature, season, time of day, reflections to name just a few), each with their own distinct effect on color. Most of us know that the first hour after sunrise and the last hour before sunset is the most ideal for photography. When the sun is low, the light passes through more of the atmosphere before it hits (your part of) the earth. When sunlight (which is more or less white because it includes all colors) hits the atmosphere, some of this light scatters (so it is diffused and not harsh). The short wavelengths scatter the most, and these are at the bluish end of the scale. In the morning and evening more blue light is lessened, causing the remaining light to have a warm red or orange color cast. During the day, the sunlight passes through less atmosphere causing more blue light to hit the earth, and this creates a cooler color cast (often bluish because of reflections from the blue sky).
Most surfaces reflect sunlight, in various amounts. Depending on these surfaces, the lighting conditions may vary greatly. The weather can be hot, cold, bright, dark, rainy, misty etc, which affect the light in their distinct ways. Even when the sun is gone at night, sunlight hits the earth via the moon (which does not produce its own light but merely reflects the sun).
The direction (position) of light:
When it comes to the direction of light, there are 360 degrees of possibilities. When the light isn't working for you, change it by moving your position, your subject's position, or the light itself, if possible.
When the light is hitting your subject from high-front (example: sunlight), it is usually the best type of light for portraitures as most of the scene will be well lit. Bright sunny days usually bring out the colors of a scene. However, if you place your subject facing the sunlight, may cause your subjects to squint. Very high sunlight (noon) will create deep shadows under eyes and chins, unless you use fill flash or a reflector.
Front lighting illuminates the portion of the subject facing the photographer. Your camera's flash is the most common type of front lighting. It is the easiest type of light to deal with photographically because there are fewer shadows to confuse the camera's light meter. Front light source however can be a bit boring—pictures lack volume and depth; textures and details are minimized and usually the scene appears flat with few shadows.
Side lighting is perfect when you want to emphasize texture, dimension, shapes, or patterns. Side lighting reveals contours and textures and exaggerates dimension and depth. At a 45-degree angle to the side, it's one of the most flattering types of portrait lighting. Side lighting usually separates the subject from the background and conveys depth and texture. The disadvantage however is that it may be too harsh for some subjects, creating some areas that are too bright, and some that are too dark. Off course you can use fill flash and / or a reflector to compensate the dark area.
Back light or light that comes from behind your subject is by far the trickiest to use, but the dramatic results may be worth the effort. Back lighting simplifies a complicated scene by emphasizing the subject, as in a silhouette. It provides a flattering halo of light in portraits and adds strong shadows in landscapes. The cons however are lack of detail in a dark subject and lens flare resulting in low contrast and strange light spots across the picture. Using exposure compensation to overcome backlighting results in too-bright background.
When it comes to light color, source, and direction in photography, there is no right or wrong. Trial and error is key. Sometimes, a color cast than is different from the one you remember seeing in your eyes is desirable, as it contributes to a certain mood in a photo. Try different White Balance settings for example: try Fluorescent when shooting snow; it will give the photo a sense of dawn. Whenever you walk around in the evening, with or without your camera, train yourself on seeing the colors on different light sources around you. Happy shooting!