When electrical is "used by a light bulb", what really happens to energy

It gets converted from one form to other forms.

The great thing about the incandescent light bulb ... the original kind,
invented by Thomas Edison ... is that it can take electrical energy and
instantly turn it into light energy and heat energy.  So you can use the
bulbs in your house to see at night without burning candles or oil, and
you can even use them in your chicken coop to keep the chickens warm.  

I used one a few years ago when I found a tiny abandoned baby possum under the bush in my front yard.  The poor thing was almost frozen.  I put
it in a box with a few of my old T-shirts, and put an incandescent light bulb
under the box to keep it warm.  In the morning, I took it to a wildlife rescue place on the lake-shore here in Chicago.  They told me that the baby would
not have survived without the heat overnight.  And three months later, they
sent me an email saying that my baby possum had grown big and strong,
and that they took him to the forest preserve and released him in the wild.

That was a rare occasion.  We don't actually use light bulbs for heat too
much any more, so it's considered wasted energy in light bulbs. We have
better ways to generate heat now, and for light, we're changing to devices
that run cool and make a lot more light with the same amount of electrical
energy ... like fluorescent tubes, CFL curly bulbs, and LED bulbs.  The
incandescent bulb is going away. 

But the principal is still the same ... convert electrical energy into light energy.