This may sound very dumb, but can anyone tell
me what castor sugar is?

And what is a brown onion/not browned...a brown onion.

I have never heard of either one..please help
Thanks

Castor sugar is usually called "superfine" sugar in the States. It's a smaller grain than regular old granulated sugar.

From what I can gather from the 'net, a brown onion is just a type of yellow onion, just a little darker.