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' cloaca, 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 -- the one that gives some breeds like Barred Plymouth Rocks and Dominiques their stripey appearance -- is passed on differently depending on whether the offspring is male or female. 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 differently-colored males and females are usually referred to as sex-links."
Here, for example, you see a trio of Golden Buff chicks. The male chick is yellow, while the two females are reddish.
But there are also some actual breeds developed that breed true and naturally produce offspring that can be told apart at hatch (versus "hybrid" breeds). Those breeds are usually referred to as auto-sexing, too 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.