Array question about [,] vs [][]

Hi,

I'm wondering what the technical difference between an array using [,] and an array using [][] is. As far as I can tell the big difference is that the [][] type is alot more complicated to setup, but I'm sure I'm missing the big picture.

Does anyone know? And why would you use [][] instead of [,]?

A comparison:

Strict

Local a:Int[5,5]
Local b:Int[][] = [New Int[5],New Int[5],New Int[5],New Int[5],New Int[5]]

For Local i:Int = 0 To 4
	For Local j:Int = 0 To 4
		a[i,j] = i + j
		b[i][j] = i + j
	Next
Next

For Local i:Int = 0 To 4
	For Local j:Int = 0 To 4
		Print a[i,j]+" vs. "+b[i][j]
	Next
Next