Here's a couple links...
https://www.backyardchickens.com/web/viewblog.php?id=1642-ISA_Browns
http://www.feathersite.com/Poultry/CGP/Sex-links/BRKSexLink.html
They save hatcheries a lot of cash because they can easily identify the sexes VERY quickly, fewer complaints from customers, less shipping cost etc.
Keep in mind, every hatchery has their own name for them. ISA browns, Golden Comets, Red Sex Links, Gold Sex Links, Cinnamon Queen mmmm,, I'm probably missing some, but they can be slightly different crosses, but those are brown egg layers, can be crossed with a Red Sire and a hen that's mostly white I think...
There's also Black Sex Linked and California Whites, which are a different cross... Pretty sure the BSLs lay brown eggs too, but CaliWhites lay white eggs.
[edit] they're quite hardy, good egg layers, but they don't breed true, you'll get something different if you hatch any eggs from them... I've had some of the Golden Comets from Mt Healthy Hatchery and they're very nice birds, but I preferr pure breeds just cause I can! LOL