Can someone please explain this to me?

I'm Scottish, born and bred, and I support the mighty Glasgow Rangers. I've always tried to garner an interest, however small, in the MLS. However, I don't understant the expansion process. I don't understand how the teams are chosen or how the teams move into the conferences or what... Could someone please explain the expansion process and why it happens. Also how do the two conferences balance out? I'm truly befuddled.