Multi-Dimensional Arrays

Rectangular Arrays - These are really 2 dimensional. The number of rows is identical to the number of columns.
Jagged Arrays - These are not really 2 dimensional and represent a 1 dimensional array of 1 dimensional arrays. Each element in the main array can hold an array of varying length.

Arrays with mutiple dimensions are also referred to as jagged arrays.

string[,] asMulti = new string[5,5]; 

Conceptually, a multidimensional array with two dimensions resembles a grid. A multidimensional array with three dimensions resembles a cube.

int[,] array2D = new int[2,3]; 
int[,] array2D2 = { {1, 2, 3}, {4, 5, 6} }; 

for (int i=0; i<2; i++) 
{ 
    for (int j=0; j<3; j++) 
    { 
        array2D[i,j] = (i + 1) * (j + 1); 
    } 
} 

for (int i=0; i<2; i++) 
{ 
    for (int j=0; j<3; j++) 
    { 
        System.Console.Write(array2D[i,j]); 
    } 
    System.Console.WriteLine(); 
} 

int[] numbers = { 4, 5, 6, 1, 2, 3, -2, -1, 0 }; 
foreach (int i in numbers) 
{ 
    System.Console.Write("{0} ", i); 
} 

Matrices

Arrays that have two dimensions are called matrices Rectangular arrays of numbers are called matrices

Jagged Arrays

A jagged array is an array whose elements are arrays, possibly of different sizes.
A jagged array is sometimes called an "array of arrays."

link - learn.microsoft.com/en-us/dotnet/csharp/programming-guide/arrays/jagged-arrays 

A variation of the multidimensional array is the jagged array: an array of arrays.
A jagged array is a single-dimensional array, and each element is itself an array.
The element arrays are not required to all be of the same size.

You declare a jagged array like this:

int[][] jaggedArray = new int[3][]; 

jaggedArray[0] = new int[5]; 
jaggedArray[1] = new int[4]; 
jaggedArray[2] = new int[2]; 

VB.Net - Syntax

Dim asMulti(5,5) As String 

GetLowerBound - Get the upper bound of the specified dimension. Starting at the number 0.
GetUpperBound - Get the lower bound of the specified dimension. Starting at the number 0.

Dim asTableValues(10, 200) As String 
Dim asTableValues As String() 

asTableValues.GetUpperBound(0) = 10 
asTableValues.GetUpperBound(1) = 200 

Dim icolumnnumber As Integer 
For icolumnnumber = 0 To asArrayName.GetUpperBound(0) 

Next icolumnnumber 

Dim irowcount As Integer 
For irowcount = 0 To asArrayName.GetUpperBound(1) 

Next irowcount