Q: What is sexing, autosexing, and sex-linked?
A: The word "sexing" simply refers to the process by which chicks are sorted into males and females. Chicks are not easy to tell apart because they don't have external sex organs like puppies are kittens... or cows, goats, sheep, horses, etc. So only experts with years of special training can "vent sex." They do that by looking at minute details of the birds' sphincters--and even then there's a chance that a baby chick can be sexed incorrectly. Read the related questions for details.
"Autosexing" refers to breeds or varieties of chickens where the males and females hatch out with different markings or different colors. Depending on the breed, a male may have a spot on his head while a female doesn't, or the males and females may just hatch out a different color.
They can do that because of "sex-linked" characteristics. Some genes are carried differently by male chickens than female chickens. For instance, the barring gene---that's the one that gives some breeds like Barred Plymouth Rocks and Dominiques their stripey appearance--is passed on differently. Males only get one copy of that gene, while females get two. So that gene is often used in hybrid varieties to produce chickens that can be told apart by the color or markings on their down.Hybrid varieties that hatch out differently marked or colored males and females are usually referred to as "sex-links."
Here, for example, you see a pair of Carolina Blue Sex-Links. The male chick has a light spot on his head, while the female has no spot.
But there are also some actual breeds developed that breed true and naturally produce off-spring that can be told apart at hatch. Those breeds are usually referred to as "auto-sexing." Technically, sex-links are also autosexing, and autosexing breeds are also sex-links. But for whatever reason, "autosexing" is a term usually reserved for breeds, while "sex-links" is a term usually reserved for hybrids.