How are foreign currencies quoted and priced?

Currencies are designated by three letter symbols. The standard
symbols for some of the most commonly traded currencies are:
EUR Euros
USD United States dollar
CAD Canadian dollar
GBP British pound
JPY Japanese yen
AUD Australian dollar
CHF Swiss franc
Forex transactions are quoted in pairs because you are buying one
currency while selling another. The first currency is the base currency
and the second currency is the quote currency. The price, or
rate, that is quoted is the amount of the second currency required
to purchase one unit of the first currency. For example, if
EUR/USD has an ask price of 1.2178, you can buy one Euro for
1.2178 US dollars.

Currency pairs are often quoted as bid-ask spreads. The first part
of the quote is the amount of the quote currency you will receive
in exchange for one unit of the base currency (the bid price) and
the second part of the quote is the amount of the quote currency
you must spend for one unit of the base currency (the ask or offer
price). In other words, a EUR/USD spread of 1.2170/1.2178
means that you can sell one Euro for $1.2170 and buy one Euro
for $1.2178.
A dealer may not quote the full exchange rate for both sides of the
spread. For example, the EUR/USD spread discussed above could
be quoted as 1.2170/78. The customer should understand that the
first three numbers are the same for both sides of the spread.