There are 3 main aspects to consider in a light source: the color, the strength, and the direction.

The color of the light source refers to the specific color spectrum. Natural sunlight has a very different spectrum than fluorescent white lamps. Natural light tends to produce great photos for many reasons, one of them is that it’s a spectrum of light that we are used to appreciate; it feels natural. Of course, this varies with Sun’s position and local weather: the mid-day sun is “cooler” (more blue), specially when it’s cloudy, but sunrise and sunset are very “warm” (more red). Color is a major component to convey a specific mood to the picture.

The strength of the light source refers to quantity and diffusion. A strong light will affect how much and how sharp the shadows will be, in contrast with the highlights of the image (the areas that receive more light). Too much contrast can be hard to capture, so softer light is usually more suitable for photography. The sunrise and sunset are special also because the sunlight becomes much softer. It’s possible to soften a strong light source with a diffusing material in-between, which not only decreases the amount of light but also spreads the outgoing ray angles a bit.

The direction of the light source refers to the position of the source relative to the photography subjects and the resulting selection of the subjects surface that receives that light. This allows the control of which shapes and textures to reveal and where the highlights and shadows should be. One more time the sunrise and sunset are notable because the Sun being closer to the ground provides more options to play with light direction.

These are all variables that can be tweaked for a single light source, but many sources can be used to compose a single shot. They can be of different colors, strengths and directions to provide a different type of contrast to the image.