Usually the device manufacturer provides an
API
for interfacing the device with the
PC
(read the documentation). Very often (for low speed devices) they provide a serial port emulator, that is you communicate with the device using standard
RS232
communication (again, check out your device documentation).
As about
Windows API
:
Windows
is the
Operative System, the
API
it provides are not only the ones used for 'the windows'. Specifically serial communication (
USB
included) functions are ready-to-use for console applications.
[update]
If you are using the
FTDI
chip a chip then (assuming the corrensponding driver is installed on your PC) you use
standard RS232 API
for communicating with it (see for instance:
"Using Communication Resources" at MSDN[
^]) on the PC side.
On the other side you typically have a, hopefully, a microcontroller with
UART
device.
[/update]