PDF (Adobe Digital Editions DRM)
ePub (Adobe Digital Editions DRM)
Mobi (Mobipocket Reader DRM)
I'd say ePubs are the most common from what I've seen. PDFs are still used a fair bit when it comes to non-fiction titles reliant on their layout. BC Library to Go seems to use PDF for nearly everything though.
And you shouldn't feel stupid for not knowing something. That's why the forum's here after all.