Bantams are smaller sized chickies. Regular sized chickens are often referred to as "standards" or "large fowl". Bantams are typically anywhere between 1/3 and 1/2 the size of the larger bird, but vary. While these little chickies may be small, many of them exhibit traits that are prized in backyard flocks...such as their gentle nature, ability to go broody, egg laying abilities and friendliness. Each breed has different characteristics, so be sure to read about them first.
Sometimes it is hard to mix bantams with standards, and many times people will have one coop and run dedicated to standards, and one for bantams.