How powerful is an on-camera (built-in) flash?

A Nikon D3200 in this case, but I expect most DSLRs are about the same. And please could I have a comparison to the light of a candle or car headlights or something.

Google 'Guide Number'

Pop Up flshes will normally have Guide Number 10 to 12

http://www.digital-slr-guide.com/flash-guide-number.html

I highly suggest you not use the pop up flash. In my opinion, it's terrible. However, creative people may find a use for it.
If you are looking for a cheap flash, have a look at the Yongnuo 568EX. It's very cheap and has many good reviews!

Your pop-up flash has a range of 10-15 meters depending on ISO and lens aperture.

Comparing it to a candle or car headlight is meaningless because it produces a very bright but very short duration (around 1/10,000th second) burst of light.

A candle.

Built-in flash adds to the price of the camera, but to be small enough to fit, power has to be sacrificed.

A typical candle gives out 1 lumen.
Your ordinary 100W lightbulb gives out 1,600 lumens.
A car's headlights is around 3,000 lumens strong.
The split-second flash from a camera is roughly 30,000 lumens.

Much less powerful than an add-on flash, by factors around 4 to 15 times, depending on model of camera and model of add-on flash.

To make comparison with continuous sources of light such as candle or car headlights you would need to specify the range of shutter speeds that you would be prepared to use - maybe around 1/60th down to 1/15th hand-held, down as low as you like (1 second, even longer?) if the cameras is on a tripod. Also you would need to specify how high the ISO could be, noting that very high ISO leads to graininess - but at different points of acceptibility in different camera models.

The time duration of a typical flash is around 1/1000th second, thus much faster than any shutter speed practical in low light conditions, thus you can't really make comparisons between any flash and any continuous source - even the Sun is a "continuous source".

All in all, no meaningful direct comparison can be given. You can note that candles produce very little light, and that car headlights produce very well controlled beams of light, thus not good for illuminating wide areas. You should also note that built-in or on-camera flash is a very "unnatural" source of light, in that in normal circumstances you do not illuminate a scene with a source very close to your eyes.