Pretty simple question, right? The answer can be pretty complex with a lot of little tangents to it.
Let's touch on just a few of the very broad possibilities here:
News Paper Photographer/Photojournalist
On-Location Wedding Photographer
Specialized Portrait Photographer
Travel Landscape and Tourism Photographer
Real Estate Photographer
Glamor/High Fashion Photographer
I am pretty sure I missed a whole lot of gray areas and maybe even some big black and white areas in there. There are a lot of possibilities in this profession. All of them have pro's and con's to them. All of them require some education in photography and some specific needs for that specialization.
Most areas of specialization will require certain equipment to be able to photograph the subjects on a professional level. Not saying you have to have the top of the line equipment, but going into a dark church wedding where no flash is allowed with a kit lens on a Canon Digital Rebel is probably going to cost you your business. It's just not possible to do that scene with that scenario and produce professional grade images from the wedding. You are going to need a camera that handles extremely high ISO's and a couple of f/2.8 zoom lenses.
Likewise you can't go photograph a home for a realtor with a 200mm lens on a crop sensor camera.
Contracting and legal requirements for each specialization are different. Model releases are definitely different for dogs and people. You wouldn't have a dog put it's paw print on your model release and you wouldn't have a portrait client sign a wedding contract. Wedding contracts are very detailed and have a LOT of information in them to cover both the photographer's interests as well as the clients'.
Marketing and branding are very different birds for each specialization. A wedding photographer probably wouldn't advertise by placing work in a veterinarian's office and a pet photographer probably wouldn't attend the local bridal fair looking for clients...
You can see where I am going with this. Let's take this week to explore some of the big specialization areas and gather some of the information for your business plan.