The format of the icons is GIF and the background is white. As you had already guessed, if you want to enjoy them on a page with a black background, you'll have to edit them.
Most of the icons possess limited dimensions, so you'll need to try them to see which one fits on your website best.
The icons are organized in folders, Instead of being organized by theme, they are organized by the use they have been given inside the developing platform of Eclipse.
There are many types of icons. Most of them are very useful, although some of them are so complex that you will surely not understand their meaning.