Commercial hybrids is a term given to cross-bred chickens bred for a specific purpose, Most hybrids available for backyard chicks are egg laying hybrids based on Rhode islands. Hybrids were first developed after the 2nd world war when poultry was first raised intensively. It was a response to the huge increase in demand for meat and eggs and was part of the agricultural revolution that swept through farming after the war. Commercial hybrids are also auto sexing meaning you can weed out the males of egg laying flocks at hatching.
The Black Rock is a hybrid between the Rhode Island Red and the Barred Plymouth Rock and is an egg laying hybrid producing about 300 eggs per year from a compact and thrifty bird.
We have the amber star hybrid (this is cappuccino on her nest), which is also based on the Rhode island stock. The birds are slight being very feathery and lays a large eggs every day (343 in her first season).
There are pros and cons to hybrids, on the plus side they reliably lay a lot of eggs and rarely go broody, they are also easy to look after and tend to be good natured. They are also available all year round and easy to get hold of.
On the negative side they rarely lay into a third season and can be very short lived, are prone to egg and bone problems, not really tame and are voracious eaters, being relatively poor foragers even in a free range environment. If you look closely at her foot in the second picture you can see she has some joint problems in her foot. There seem to be common problems related to growing to maturity too fast.