What happens if I delete an album? Is there an undo?

Deleting an album removes it permanently — the button on the album edit page doesn't mince words: "Delete Album — remove this album permanently." There's no recycle bin mentioned anywhere, so treat it as final. Before you go deleting old work, download anything you might ever need (the file manager has a Download Album option for exactly this), and double-check you're not still using the album for an active client. When in doubt, keep it in cold storage instead.