Not remotely an expert, but the basic idea is that hobbyist pilots are licensed to fly for their personal entertainment/transportation only.

They are not licensed as airlines, so selling tickets at a profit is a definite no-no. But they can take their friends along for the ride and split the cost of fuel.

A number of apps/websites have popped up that blur the line between "operating an illegal airline" and "doing the equivalent of a carpool in a Cessna." That's the gray area that needs to be fleshed out.