How do I run a macro every time a certain cell changes its value?

There is an event called Worksheet_Change which is triggered when a value is entered (it will not fire when a formula result changes). One of the arguments to this event is 'Target' which is a reference to what changed. Since this event will occur whenever a value changes - you can use the target to see if it is the cell you are interested in:

Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("C5")) Is Nothing Then
Exit Sub
Else
'The cell you are monitoring has changed!
'Do whatever you need to do...
End If
End Sub