There are two types of Heartless: Emblem Heartless and Pureblood Heartless.
Emblem Heartless, on the other hand, are created from machines that reproduce the process of a heart being consumed by darkness. They have more varied shapes, colors, and are branded with Xehanort's symbol to differentiate them from the Pureblood ones. They mostly appear in the many worlds that reside in the realm of light, unlike natural born Heartless.
Pureblood Heartless are different from Emblem Heartless because they lack the symbols seen on Emblem Heartless. Each pureblood Heartless is the accumulated darkness of a destroyed heart. They come in every shape and size, but it is a rule that the more powerful they are, the more monstrous they get. They seek relentlessly to consume a lot of hearts, and will stop at nothing to consume just one. When they consume a heart, they destroy it, creating another Heartless.