how are interfaces created? (talking about the Java-like "interface")

[Ben Johnson <ben blarg net>]

I want to understand how interfaces are defined and used/implemented.
The docs say, for instance:

"GtkButton implements AtkImplementorIface"

Is AtkImplementorIface a real thing?  How does the GtkButton implement
it?  I haven't been able to find docs about this.  Can someone point me
in the right direction?  Are interfaces often or ever defined by GTK+
users like me?

Thanks,

- Ben