How do foreign currencies gain or loose against the dollar?

I'd like to know if hearing that a foreign currency is strong/weak against the dollar is good/bad for the dollar and what determines the exchange rate.

The valued of the dollar against any foreign currency is dictated by supply and demand for dollars in terms of that currency. For example, right now, the dollar is weak against the euro. This simply means that there is more demand for euros than for dollars. Whether this is good or bad depends on your perspective. If you are a European tourist in the U.S., it is good — you will be able to obtain meals, transportation, hotel rooms, etc., cheaper in terms of your own currency. If you are a U.S. resident, however, and want to buy, for example, a car made in Europe, it is bad — that vehicle will be more expensive in terms of dollars. However, a weak dollar is good for multinational Companies that do business overseas — say GM or MacDonald's — because their products will be bargains for overseas buyers. So when the dollar is falling, it is often a good bet to buy the stocks of multinational corporations based in the U.S.