Hi,
the Airtrain in JFK is very nice system and very easy to use. There are two lines:
One line is a circle line and connects clockwise all terminals of the airport. This line is for free and runs every 2 - 4 minutes.
The other line connects anticlockwise all terminals with the long parking lot and the subway stations "Jamaica station" and "Howard Beach". This line costs about 5 US$. If you use this line only for travelling between the terminals, it is for free.
JFK Airtrain
Displays at the stations and in the train shows you the destination and the next stop. There are also automtic announcements.
Best regards,
taigatrommel