When you use Catch on it's own, it catches every exception that is thrown (and discards all the information on what happened at the same time).
Dim Top As Double = 5
Dim Bottom As Double = 0
Dim Result As Integer = 0
Try
Result = Top / Bottom
Catch ex As System.OverflowException
MessageBox("Attempt to divide by zero:" + ex.Message)
End Try
Restricts the Catch to just the exceptions you can do something about.