A billet lower is also machined from a solid block of aluminum - called a billet - however the entire process of making a billet lower is something like creating a sculpture. This process doesn't involve large forging hammers pounding out the shape, but rather material is removed from the billet slowly and methodically until all that remains is a functioning lower receiver. Billet lower manufacturers take advantage of this manufacturing process to get creative with the design of their lowers and the results can be pretty awesome albeit more expensive.