Each airline makes their own decision on the type of power source they
provide, and where they place the outlets in their aircraft. Seatguru
is a good place to see what is available on various aircraft and
airlines. Here is the link to a typical arrangment in American
Airlines' 777 fleet:
http://tinyurl.com/bmdo4
You can tell where the power points are by the dots on the seat chart.
You can get more information on the type by clicking on the image of the
plug toward the top of the page.
Here is a link to a general discussion of the various types of onboard
power sources:
http://www.seatguru.com/articles/in-seat_laptop_power.php
In the case of American's 777, they offer a standard cigarette style
jack, like you would find in an automobile. It is rated at 15 volts DC,
and 75 Watts. Therefore, anything that you could plug into your car
cigarette lighter, that draws less than 75 Watts (5 Amps), can be used
on the aircraft.