There are entire classes taught on this, but essentially the White Army was anyone who was not communist, so Tsar loyalists, people in favor of a democratic system, etc.

As for why the Civil War happened, after the Tsar was deposed, there was a power vacuum left behind that needed to be filled and different groups had different ideas about how it should be done.

As a disclaimer, I am working from memory here, so take this with a grain of salt. I'm sure I missed some of the finer details, at least.