Lighting
Apart from film, cameras and lenses, the only other necessary component for photography is light. 
There are two kinds of this, natural and artificial. Natural light is by far the brightest and most abundant of the two, but it tends to go away daily (unless you live at the north pole) and so occasionally photographers turn to artificial lighting.
My preferred artificial light source is studio flashes. These provide a great amount of light for a very brief time, without too much heat being generated (the alternative to studio flash is tungsten lighting, which can serve as an alternate heating source for a small city). Studio flash is the same colour temperature as daylight, and can be controlled very accurately in a well equipped studio, giving an almost unlimited number of scenarios for lighting a subject. The only downside of studio flash is that it is expensive. When I attended art college, I had generous access to their lighting studio, which included a very powerful flash kit. It was heaven, and though I still have access to studio flashes, none of them compare to what I used in school.
I currently own a White Lightning studio system, consisting of three X1600 flash heads, stands and a variety of soft-boxes and other light modifiers. 
This system is more powerful than I need for my figure work, but as I also plan to use it for other subjects (specifically dance photography) having a system with considerable light output was important when planning for the system. In addition to the studio lights, I use a couple of reflectors, and have two sets of background - one white and one black (both are 8’x24’ in size). These are somtimes supported by a background stand, but as often as not, I clip them to doors, window frames and furniture.
The real art of photography is in working with light, natural or artificial. Too many people take light for granted and focus on the pose and setting - both of which can be empty without the right light. On the other hand, even the blandest room, or the most simple pose can be transformed and elevated by good lighting.