Sunlight
Different types of sunlight; the benefits & drawbacks of each
Other factors related to the type of sunlight are the intensity of the light, the duration of exposure, and the overall quality of the light. The closer to the light source, the more intense the light. So plants that need a lot of light for a long period of time need to be planted in bright areas - such as along a southern window.
The duration of light is more important to some plants (especially those that only flower during the short days of winter) than others.
The quality of the light is also important (but this explanation gets into wavelengths, which gets into physics, which gets confusing so we'll have to skip that here!). Light quality affects plants because photosynthesis requires certain wavelengths, while flowering and fruiting require others.