Bacalao is dried and salted cod, from which all moisture has been removed. Whilst this refers specifically to the dried fish, there is no other word in the Spanish language for cod as before the advent of refrigeration, fresh cod did not exist. Salt cod has quite a history and infact, I once read an entire book on the subject and, believe it or not, it was most entertaining. Whilst it is obviously no longer required as a means of preservation, bacalao is still popular due to its unique flavour and texture.
Salt cod is normally sold as loin or fillets, but sometimes trimmings are available and these are much cheaper and suitable for some recipes. It needs to be soaked for up to 48 hours with frequent water changes to remove the salt. It can then be used like normal cod, but it features in many traditional recipes, notably from the Basque Country, which showcase its individual propeties and many people prefer it to fresh fish.