Bantams are the "fun-sized" equivalent of a number of other breeds. This comes with many caveats, but it is safe to assume that when someone says "bantam," they mean a bird one-fourth the size of most others.
True Bantams: are breeds with no large counterpart. They make look like larger breeds but are wholly unique.
Miniature Bantams: have a large counterpart.
Developed Bantams: are bred for a specialized purpose. All miniatures are developed. Some true bantams are developed.