How are interfaces defined and used in the Go language?

In Go, interfaces are defined as a set of methods that a type must implement to be considered satisfying that interface. The interface definition looks like this:

// Interface definition
type Reader interface {
    Read(p []byte) (n int, err error)
}

A type automatically implements an interface if it has all the methods specified in the interface. There is no need to explicitly declare implementation.

Example usage:

type File struct {}

func (f File) Read(p []byte) (n int, err error) {
    // read implementation
    return 0, nil
}

func process(r Reader) {
    buf := make([]byte, 100)
    r.Read(buf)
}

func main() {
    var f File
    process(f) // File implements Reader
}

Thus, interfaces in Go allow writing flexible and extensible code, relying on the behavior of types rather than their specific implementations.