const are compile time constants where as readonly are runtime constants. Once assigned you cannot change the values assigned to variables declared as const or readonly.

Usage:

Const variable just like their static(shared in VB) counterparts are accessible via Class Names only. They are not accessible via objects.

Readonly variable are instance variable and accessible via objects.

C#  

internal sealed class Program
    {
        const int iConst =20;
        readonly int iReadonly;
        
        public Program(int iReadonly)
        {
            // Readonly variable can be assigned a value from configuration file or database
            this.iReadonly = iReadonly;
        }
    }

Usage:

Program programObject = new Program();
            Program.iConst =20;
            //programObject.iConst =20;  Compile time error
            programObject.iReadonly =20;

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VB.Net

Friend NotInheritable Class Program
Const iConst As Integer = 20
ReadOnly iReadonly As Integer

Public Sub New(iReadonly As Integer)
' Readonly variable can be assigned a value from configuration file or database
Me.iReadonly = iReadonly
End Sub
End Class

Usage:

Dim programObject As New Program()
Program.iConst = 20
'programObject.iConst =20;  Compile time error
programObject.iReadonly = 20