FOREX currencies are traded in much smaller divisions than cash. Whereas the smallest division in US cash is the penny ($0.01), US currency can be traded on the FOREX in divisions of $0.0001. This smallest division is called the pip (short for Price Interest Point – sometimes just called 'points').
Since currencies are traded in large lots of (say) $100,000 - small movements in value can generate substantial profits and losses. In a lot of US$100,000 one pip is worth $10 so an increase in 40 pips (4/10 of one cent) can generate a profit or loss of $400.
Currencies are traded in lots of various sizes. The standard lot is 100,000 units of the base currency. A unit is the currency name e.g. one unit of US dollars is the dollar. So a standard lot of US currency is worth $100,000. FOREX trades can have lots of various sizes - a mini lot is 10,000 units, but the most trades are done using standard lots.
Various currencies have different sized pips. The US dollar is expressed in pips of 0.0001 while the Japanese yen is expressed in pips of 0.01. The value of a pip depends on the size of a lot and the currency pair traded. Currency pairs with USD as the quote (second) currency (e.g. CAD/USD) always have a pip value of $10 per standard lot or $1 per mini lot. A pip value calculator can be used to calculate other currencies.