A Plain Vanilla Call is the right (note: not the duty) to buy a
certain amount of a currency at a specified time at a specified exchange rate.
In our tool we restrict ourselves to the European Style of that option, which means
that the holder of the option cannot buy the specified currency at any time earlier
than previously agreed. To put it another way, the holder of the option cannot exercise
the option before the specified expiry time in order to buy the agreed currency
at the agreed exchange rate (i.e. the specified strike price).
The other version of that option is an American Call, which can
be exercised any time before expiry.
A Put option works in the same way as the Call option, with the
difference being that now the holder has the right to sell a currency at an agreed
exchange rate at expiry.