[go-nuts] difference between capacity and length in slices?

Hi! I'm going through tour.golang.org
I'm on #34. It mentions you can specify the capacity in addition to the
length when making a new slice.

What are the difference between the two? I couldn't find the answer
anywhere.

package main

import "fmt"

func main() {

     a := make([]int, 5)
     a[0]=1
     printSlice("a", a)
     b := make([]int, 0, 5)

     printSlice("b", b)
     c := b[:2]
     printSlice("c", c)
     d := c[2:5]
     printSlice("d", d)

}

func printSlice(s string, x []int) {
     fmt.Printf("%s len=%d cap=%d %v\n",
         s, len(x), cap(x), x)
}


a len=5 cap=5 [1 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0]
d len=3 cap=3 [0 0 0]