In essence, what you will do is to help connect the people who want to buy a product or service with those who want to sell a product or service.
Your referral business can cover virtually anything you can imagine, from a business where dentists sign up to get referrals from clients (like the old 1800 Dentist program) to a program where you help connect people who need a writer to the writers willing to do the job (like Text Brokers).
The other option is to arrange for the referral business to have a range of specialties.
For example, if you work in a smaller area, you could provide referrals for all kinds of services and not just one particular service.
In this case, you are acting more like an advertising service than like a referral business, but the concept is basically the same.
You, as a trusted source are recommending people for their services.
That last point is probably the most important thing to remember about running a referral business.