Pretty much the same deal. Most of them started as an offshoot of local boards of education and 1960s experiments in using televsion as an educational medium. Then, at some point, they were spun off into their own not-for-profit/foundation entities. Like the NPR stations, they pay the network for their programming, partly with federal grant money. In Los Angeles recently, the main PBS station decided it didn't want to pay anymore and dropped out of the system.