When portrait photographs are composed and captured in a studio, the photographer has control over the lighting of the composition of the subject and can adjust direction and intensity of light. There are many ways to light a subject's face, but there are several common lighting plans which are easy enough to describe.
Three-point lighting
One of the most basic lighting plans is called three-point lighting. This plan uses three (and sometimes four) lights to fully model (bring out details and the three-dimensional of) the subject's features. The three main lights used in this light plan are as follows:
Key-Light
Also called a main light, the key light is usually placed to one side of the subject's face, between 30 and 60 degrees off center and a bit higher than eye level. The purpose of the Key-Light is to give shape (modelling) to a subject, typically a face. This relies on the first principle of lighting, white comes out of a plane and black goes back into a plane. The depth of shadow created by the Main-Light can be controlled with a Fill-Light.
Fill-in light
In modern photography, the fill-in light is used to control the contrast in the scene and is nearly always placed above the lens axis and is a large light source (think of the sky behind your head when taking a photograph). As the amount of light is less than the key-light (main-light), the fill acts by lifting the shadows only (particularly relevant in digital photography where the noise lives in the shadows). It is true to say that light bounces around a room and fills in the shadows but this does not mean that a fill-light should be placed opposite a key-light (main-light) and it does not soften shadows, it lifts them. The relative intensity (ratio) of the Key-light to the fill-light is most easily discussed in terms of "Stops" difference (where a Stop is a doubling or halving of the intensity of light). A 2 Stop reduction in intensity for the Fill-Light would be a typical start point to maintain dimensional (modelling) in a portrait (head and shoulder) shot.
Accent-Light
Accent-lights serve the purpose of accentuating a subject. Typically an Accent-light will separate a subject from a background. Examples would be a light shining onto a subject's hair to add a rim effect or shining onto a background to lift the tones of a background. There can be many accent lights in a shot, another example would be a spotlight on a handbag in a fashion shot. When used for separation, i.e. a hair-light, the light should not be more dominant than the main light for general use. Think in terms of a "Kiss of moonlight", rather than a "Strike of lightning", although there are no "should" in photography and it is up to the photographer to decide on the authorship of their shot.