First, there is an INPUT tag that causes a button named "BtnHello" to be created.
<INPUT TYPE=BUTTON VALUE="Click me" NAME="BtnHello">Second, there is a script tag that contains an event handler for BtnHello's "OnClick" event. As in Visual Basic, event handlers are named using the pattern ObjectName_EventName, which in this case means BtnHello_OnClick. When the button is clicked, the procedure with that name is run. In response to the button being clicked the event handler puts up a message box by using VBScript's MsgBox runtime function.
<SCRIPT LANGUAGE="VBScript"> Sub BtnHello_OnClick MsgBox "Hello, world!", 0, "My first active document" End Sub </SCRIPT>