There ARE a lot of different terms for chicken, aren't there? It can be a little confusing, especially when you're just starting out. So let's define these terms.
Male and female chicken terms
Hen, rooster, roo, capon, chicks, peeps: You probably know the terms "hen" and "rooster," which refer to female chickens and male chickens respectively. "Roo" is just short for "rooster," and "capon" refers to a neutered rooster. What you may not know is that we don't normally refer to young chickens or chicks by those terms. A baby chick is not a hen or a rooster. "Hen" and "rooster" are terms used to refer to adult chickens only. By contrast, baby chickens of either sex are called "chicks," but can also be called "peeps." Why aren't there different terms for female baby chicks and male baby chicks? Probably because historically, it was many weeks before it was possible to tell them apart. The Western world didn't know how to tell male chicks from female chicks until the 1930s, when we learned about it from the Japanese.
Chicken, rooster: Sometimes newbies get confused and think that "chicken" means female, and "rooster" means male. So we sometimes hear people say things like, "I have ten chickens and two roosters." What they mean is that they have a total of 12 birds: ten hens, two roos. What they're saying is that they have 10 chickens, two of which are males and eight of which are females. So, if you're a newbie, be sure that you're clear on the fact that "chickens" refers to BOTH males and females.
Pullets, cockerels, juveniles: When they're young, female chickens are "pullets," and male chickens are "cockerels." Young chickens of both sexes--pullets and cockerels--can be called "juveniles" or "juvenile chickens."
So... what's the age difference between a chick, and a juvenile, pullet or cockerel? It's a little fluid, but generally baby chickens go from being called chicks to being called pullets or cockerels once they grow in feathers rather than down. Male chickens go from cockerels to roosters once they hit puberty and begin mating; female chickens go from pullets to hens once they hit puberty and begin laying. If you're not confused enough, yet, we should add that sometimes female chickens are called "pullets" for more or less their entire first year, even after they begin laying! This is because when they first begin laying, their eggs are not full size. (They start small--lucky for them!) So "pullet eggs" refers to small eggs laid by young female chickens.
Started pullet or started cockerel: These are more specific terms you'll often hear used by hatcheries or breeders. In this context, "started" just refers to the fact that someone has started raising them already. If you buy started birds, you won't start with them as chicks; you'll start with them as pullets or cockerels. That said, you don't normally talk about your own birds as started pullets or started cockerels--not unless you just bought them as starteds, or unless you plan to sell them yourself. "Started" is more of a business term. Started pullets can sometimes be "point-of-lay" pullets, too, meaning the chickens are four or five months old, and just about ready to begin laying eggs.
Broody: "Broody" in this sense is just fancy jargon for a mother hen. A "broody" is a hen who is either setting on eggs to hatch them, or has hatched them already and is raising the chicks.
Biddy: This is a colloquial term you'll hear from time to time that refers to female chickens. Originally, it probably referred specifically to an older hen (it also referred to an older woman, especially a querulous old woman or busy-body--comparing her to chicken was pejorative). Later it came to refer to juvenile OR mature chickens. The word likely derived from sounds made to call the flock "biddy-biddy-biddy." But today, we have even seen people mistake the spelling--"bitty" or "bittie"--and consequently believe it's a term that refers to "itty bitty" baby chicks. Perhaps the meaning will eventually shift again, but for now it is spelled B-I-D-D-Y and refers to pullets or hens.
Chook: You may hear this term from time to time on chicken forums This is just UK and AU slang for "chicken." Neat!
Collective nouns for chickens
Last, let's talk a few group terms.
Flock: You doubtless know this one! It's a term that describes a group of chickens that live together. Most backyard chicken keepers probably just have the one flock. You might have two or more flocks if you keep chickens in separate enclosures, for instance, if you breed chickens. So, you might have a flock or Orpingtons and a flock of Marans. Or perhaps you have two different flocks of mixed breeds, that you keep separate for one reason or other.
Clutch: a clutch is a term used to refer to a broody's collection of eggs that she will hatch, and to just-hatched chicks when they are still drying off beneath mom, and too young to venture out.
Brood: A brood refers to a group of baby chicks that all hatched at the same time. A mother hen, or "broody," raises a "brood" of chicks. Historically, a group of chicks can be referred to as a "chattering of chicks" or a "peep of chicks." Today, it's a little more common to use the term "peep" for each chick, rather than the group! And I don't know that I've ever heard anyone refer to a chattering, unless it's in a literary/etymological context.