package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

A goroutine is a function that is capable of running concurrently with other functions. To create a goroutine we use the keyword go followed by a function invocation:

package main

import "fmt"

func f(n int) {
  for i := 0; i < 10; i++ {
    fmt.Println(n, ":", i)
  }
}

func main() {
  go f(0)
  var input string
  fmt.Scanln(&input)
}

Generally, function call executes all the statements inside the function body and return to the next line. But, with goroutines we return immediately to the next line as it don't wait for the function to complete. So, a call to a Scanln function included, otherwise the program has been exited without printing the numbers.