The term Reserve/Reserva/ Riserva normally means higher quality wine than usual, or a wine that has been aged before being sold. Traditionally, winemakers would "reserve" some of their best wine from good vintages rather than sell it immediately- hence the name.
Before a wine is called a Reserve Wine it needs to age for a minimum period of time as established in law in some countries. There is also the term Gran- Reserva which means that the wine was aged for even longer periods in casks and bottles. Usually a Reserve wine needs to age for at least three years.
Now the key for the consumer is to recognise which reserve is genuine. The Reserve status is not regulated everywhere, however wines coming from the Old World specially from Portugal or Spain can most certainly be bought with confidence as the term is highly regulated there and demands a minimum aging term of at least three years, in Italy the law demands a minimum aging period that ranges from 27months to 5 years depending on the wine.
Countries from the New World do not yet have strict Reserve status regulation and as such the Reserve status criteria is left to the be established by producer. Most of the time it does mean that a wine with the reserve status is of better quality than the original product. It just has not been subjected to strict controls as you would find in the Old World wines. In other cases Reserve it is only a marketing exercise.
So, if you are looking to buy Reserve with confidence we suggest Old World wines, however if you normally like a particular wine from the New World countries and they offer the Reserve line then it would be fair to assume that this will be a better wine than its original.