A few nice cnc aluminium machining images I found:
Subtractive vs Additive Processes
Image by Ѕolo
CNC Machining: start with a large solid block and remove material to create your part.
3D Printing/Rapid Prototyping: create the part by binding loose material layer by layer.
Two great tastes that taste great together.
Machined aluminum brick from Bram, printed Alumide brick from me via Shapeways.