Red Sex Links are hybrids produced by crossing a red gene rooster (Rhode Island Red, New Hampshire, or Production Red) with a silver gene hen (Rhode Island White, Silver Laced Wyandotte, Delaware, Light Sussex, or silver factor White Rock). The offspring can be sexed by color from hatching; male chicks are whitish, female chicks are reddish, male adult birds are primarily white with some reddish feathers in the saddle area, female adult birds are red with white underfluff and some white feathers in the tail. Red Sex Links are egg laying machines outlaying either parent breed. It's one of the interesting quirks of hybridization. Isa Brown, Lohman Brown, Red Star, and Golden Comet are four of many labels under which hatcheries market their Red Sex Links. Other RSL labels include Gold Star, Cinnamon Queen, Golden Buff, Brown Sex Link, Bovans Goldline, Red Shaver, Brown Shaver, Warrens, etc. The reason they all look the same to you is because they are essentially the same hybrid; the only real difference being which silver gene hens are used in the crosses and this makes very little difference, either in the appearance of the RSLs or in their egg laying abilities. Because Red Sex Links are hybrids, they will not breed true and the offspring of two RSLs cannot be sexed by color. There is also a Black Sex Link which is produced by crossing a red gene rooster (RIR, NH, or PR) with a barred hen (usually a Barred Rock). Like RSLs, BSLs are egg laying machines and can be sexed by color from hatching; male chicks are black with a white spot on top of their heads, female chicks are black without the white spot on top of their heads, adult males are barred, adult females are black with some reddish/copper colored feathers around the neck area. And like RSLs, BSLs will not breed true when bred to each other.
Production Reds on the other hand are produced by crossing a red gene rooster (RIR, NH, or PR) with a red gene hen (RIR, NH, or PR). As a result of crossing two red gene parents, they cannot be sexed by color from hatching as both male and female PR offspring will be reddish. The adult PRs (both male and female) will be primarily red and lack the white underfluff and white tail feathers of the RSLs. And while PRs are excellent layers, they are not typically quite the egg laying machines that RSLs (or Black Sex Links either) are. The major advantage of PRs over Sex Links is that they will breed true. Production Red roosters bred to Production Red hens will produce Production Red offspring; in other words the chicks will carry the same general appearance and qualities of their parents.
I have posted pics of both a RSL hen and a PR hen below so that you can see the difference in their appearance (note the white underfluff and white tail feathers of the RSL). Hope this helps.
Red Sex Link hen
Production Red hen