“struct sockaddr” is a generic definition. It’s used by any socket function that requires an address.
“struct sockaddr_un” (a “Unix sockets” address) is a specific kind of address family.
The more commonly seen “struct sockaddr_in” (an “Internet socket” address) is another specific kind of address family.
The cast is what allows the sockets APIs to accept a common parameter type that will actually be any one of several different actual types.
Here’s a good link that shows several different address family definitions:
http://www.cas.mcmaster.ca/~qiao/courses/cs3mh3/tutorials/socket.html