Cavalier King Charles Spaniels (not to be confused with King Charles Spaniels or, as they're known in the United States, English Toy Spaniels) are among the largest breed of toy dogs. They come in four colors: Blenheim, Ruby, Black and Tan, and Tricolor (black, tan, and white). They weigh between 13-18 pounds and stand 12-13 inches at the withers.

These dogs were common subjects of paintings in the 16th-18th centuries. The breed gets its name from King Charles II of England, who adored his dogs; he was accused of paying more attention to them than to courtly matters. Toy Spaniels of the time had a relatively long nose and flat head; gradually, they were crossbred with either Japanese Chins or Pugs (it is speculated), and the breed developed a more domed head and a flatter nose. These are the dogs that are known today as King Charles Spaniels or English Toy Spaniels.

In 1926, an American named Roswell Eldridge began to search in England for a dog exemplifying the dogs of the old paintings. He offered a prize every year at the Crufts Dog Show for the dog that most resembled those that Charles II adored. Finally, in 1928, a winner was selected (alas, after Eldridge's death). A breed standard was created based on this individual dog.

During World War II, the revival project was put on hold. When it began again in the 1950s, there were only a handful of dogs to work with. Every Cavalier today is a descendant of those few dogs. Unfortunately, this has resulted in a high rate of heart problems in the breed; when looking into purchasing a Cavalier, it is very important to confirm the dog's own health as well as check the history of its parents.

Cavaliers are extremely affectionate, companionable dogs, equally well-suited to live in an apartment or someplace with lots of room to roam. They are a good choice of a breed for the elderly, being very gentle; yet they are also active enough that they can play tirelessly with children.


Sources:
http://www.ckcsc.org/ckcsc/ckcsc_inc.nsf/Founded-1954/breedhist.html
http://www.ckcsc.org/ckcsc/ckcsc_inc.nsf/Founded-1954/standard.html
and general knowledge accumulated and mentally compiled over time from a variety of sources.