OP FYI:
Machined Rims - Rims that have had the sidewalls ground down flat for better braking. Paint or anodizing will be ground off, and the surface made flat as possible to provide better contact between the brake pad and the rim.
Non-Machined Rims - Rims with the sidewalls kept in their raw condition. If the rims are painted or anodized the sidewalls will have it as well. These rims are not as good for rim brakes, but it doesn't matter if you are riding with disc brakes or brake less.