They are two different breeds.
Barred rocks are a variety of the Plymouth Rock breed of birds - Barred refers to the bar pattern.
"Plymouth Rocks are large, long-lived chickens. Some varieties are good layers while others are bred principally for meat. They possess a long, broad back; a moderately deep, full breast; and yellow skin and legs. The hens have a deep, full abdomen, which is a sign of a good layer. The face of a Plymouth Rock is red with red ear lobes, a bright yellow beak, bay-colored eyes, and a single comb of moderate size. Their feathers are fairly loosely held but not so long as to easily tangle.
Generally, Plymouth Rocks are not aggressive, and tame quite easily. They are docile and may show broodiness. The hens usually make good mothers. However, some males and females are big and active enough to be quite a problem if they become aggressive.
Breeders should be aware of the standard weights and not select small or narrow birds for the breeding pen. Common faults include a shallow breast, high tails, narrow bodies and small size. Friendly and curious."
Dominiques are another breed entirely
"Dominiques, sometimes called Dominickers, are quite distinctive in appearance, having a rose comb and a heavy plumage of irregularly striped black-and-white feathers (a pattern called "barring" or sometimes "hawk coloring"). The breed matures quickly, producing eggs at about six months of age.
At first glance, Dominiques and Barred Rocks appear strikingly similar, often leading to confusion when discerning a particular breed. The strongest indicators are the
Comb
Dominiques possess a rose comb while Barred Rocks possess a single comb.
Plumage
Dominiques exhibit staggered barring in their plumage, lending to a somewhat mottled appearance. Barred Rocks exhibit crisp, parallel barring.
Color
Dominiques exhibit a softer contrast of "not quite black on not quite white", while Barred Rocks exhibit a high-contrast black-on-white color."
"The Dominique, also known as the Dominicker, is a breed of chicken originating in the United Kingdom. It is considered America's oldest breed of chicken, having been brought to New England from southern England during colonial times. By the 19th century, they were widely popular and were raised in many parts of the country. Dominiques are a dual purpose breed, being valued for their meat as well as for their brown eggs. They weigh 6 to 8 pounds (2.7 to 3.6 kg) at maturity. In earlier times, their feathers were much sought after as stuffing for pillows and mattresses."
A little research goes a long way - thank you Wikipedia!