Event procedures run automatically when something happens: a cell changes, a sheet is selected, the workbook opens. They live in the sheet or ThisWorkbook module and have fixed names.

Code

Private Sub Worksheet_Change(ByVal Target As Range)
  If Not Intersect(Target, Range("B:B")) Is Nothing Then ...
End Sub

• React to a change

  Private Sub Worksheet_Change(ByVal Target As Range) If Not Intersect(Target, Range("B:B")) Is Nothing Then MsgBox "B changed" End Sub → Reacts only to edits in column B

  Intersect limits the trigger.

• React to selection

  Private Sub Worksheet_SelectionChange(ByVal Target As Range) Application.StatusBar = Target.Address End Sub → Shows the selected address in the status bar

  Selection events fire constantly; keep them light.

• Block an action

  Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean) If Range("A1").Value = "" Then Cancel = True End Sub → Blocks saving until A1 is filled

  Cancel arguments stop the action.

• Event code lives in the sheet or ThisWorkbook module, not a standard module.
• Pick the object and event from the two dropdowns at the top of the module to get the exact signature.
• A Change event that writes to the sheet must disable events or it calls itself.
• Events fire for code changes too, not just user edits.

• Writing to cells inside Worksheet_Change without EnableEvents = False causes an infinite loop.
• Events do not run while macros are disabled or Application.EnableEvents is False.