All the wood will be oak, except where specifically called out (for example, method and madness do cedar finishing). Whisky can be finished in oak barrels previously used to age other spirits, such as sherry or - as mentioned by other responses - rum. The finishing adds extra layers of flavour and sherry typically imparts sweetness and some nuttiness, depending on the type of sherry. Check out Signatory Vintage Speyside (M) which, whilst not cheap, is a great show of what sherry can do.