A hard (or strong) currency is one that can be used to trade internationally and that is a stable store of value. Hard currencies are normally present in politically and economically strong countries, with stable government and low inflation. It is expected that hard currencies will not depreciate in value against other currencies, as this would be a characteristic of soft currency.
Soft currencies, conversely, are the opposite of hard currencies. A soft currency is expected to de-value compared to over time and is normally the currency of developing countries such as Zimbabwe. The Zimbabwean dollar was a good example of a soft currency, as their inflation rates were as high as 3714% per year, which was in part due to, and in part led to the printing of money. Currencies such as the Zimbabwean dollar are rarely used for international trade or for keeping large stores of.
There were fears at first that the euro would be a soft currency, but such fears were not realised and this is probably no surprise - given the nature of the economies who use it as well as the highly specific entry requirements. There also seems to be quite effective management of the currency by the European Central Bank. Their interest rates are higher than those set by the central bank in the UK, and eurozone growth is higher than that in the UK, which puts the EU in a better position than ourselves. Also, the ECB clearly doesn't want any countries to drag the currency down, such as Portugal, as they are choosing instead to take very drastic measures such as bailouts as well as setting strict fiscal policy guidelines to those countries that fail to manage their own economies successfully.