I'm not sure if anyone knew these, but I actually just ran into this - I never knew this myself either. Found this on Wikipedia.

The following terminology is used to describe horses of various ages:

Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at 4-6 months of age.
Yearling: a horse of either sex that is between one and two years old.
Colt: a male horse under the age of four.
Filly: a female horse under the age of four.
Mare: a female horse four years old and older.
Stallion: a non-castrated male horse four years old and older. Some people, particularly in the UK, refer to a stallion as a "horse."
Gelding: A castrated male horse of any age, though for convenience sake, many people also refer to a young gelding under the age of four as a "colt."

I was told the definition of a herd is 6 or more horses. Under 6 horses is a band of horses. Now you can be around cowboys and correct them when they call 4 horses a herd of horses:p

I found this information very cool. I usually just call my stallion and mare, 'horses'. From now on I'm going to try and call them by the correct terminology so that I don't confuse experienced horse owners.