toroidal power transformers?

I know a toroidal power transformer is the round, donut looking thing in my amp, but what does it do? Are some better than others? Are there alternatives? -Thanks.
changes A/C voltage. The stuff in the box needs electricity. The 120V outside is not the right 'voltage' for the stuff in the box. The transformer, the donut, changes the voltage to several (usually several) that the parts in the box can use.
Common voltages are 35V+ and 35V-, 12V+, 12V-, 6V etc.
Then SOME stuff needs direct current not A/C current so there are diodes and stuff to change it to direct current...
Every part in there does something important.
Toroidial transformers are the usual in better boxes, some new "R" Core transformers are coming out in stuff, and the 'old' standard transformer.
The best for each app is decided by the maker.
Elizabeth is correct. And yes, as with all products of the world there are quality differences. Toroidals are not necessarily better or worse than other designs. Some of the best "iron" is used in the old Dynaco 70. My understanding is that David Hafler performed extensive research on eddy currents and overall interaction with AC and transformers.
The best iron isn't iron at all, but metallic glass! It's too pricey for most transformers though...
Although this two-part article is arguably somewhat biased, it does a pretty good job of describing the tradeoffs that are involved between the leading types of power transformers:

-- Al
Al, that was somewhat biased.

Ayre and Onkyo are exceptions for their own reasons.