I'm not sure what a "hydro brake" is, but a hydraulic disc brake, as used on bicycles, is a hydraulic braking system that uses a lever containing a master cylinder, a caliper containing one or more slave cylinders, and a sealed, high pressure, hydraulic hose that connects the two. The system is filled with hydraulic fluid. Pulling the lever compresses the fluid which moves the caliper pistons which contact the disc.
On bicycles, disc brakes, no matter how they are actuated, come in several styles. For levers, they can be set up for drop bars or flat bars, with or without rise or sweep. For calipers, they can be single or multi piston, and usually either flat mount or post mount. Flat mount calipers have threaded bosses in them. The mounting hardware goes through the frame and into the caliper. Post mount calipers are the opposite. Mounting hardware goes through the caliper and into a threaded boss on the frame. Rotors are usually either 6-bolt or centerlock, which defines how they are mounted to the wheel hub. Rotor diameter depends on the frame design and rider needs. Calipers have a base rotor diameter, usually 140mm or 160mm, where they can be direct mounted to the bike. Larger rotors require adapters or spacers to properly space the caliper away from the rotor, and ensure proper brake pad contact. Pads are usually resin, semi-metallic, or metallic.
Cable actuated brake bikes can be converted to hydraulic brakes. Rotor size can be changed. Single or 2-piston systems can be converted to multi-piston. Flat mount can be converted to post mount, and sometimes, the other way around. It all depends on the bike.
As someone mentioned in another thread, these hybrid cable actuated hydraulic calipers can provide some degree of improved lever feel, consistency, and braking power while retaining the stock cable actuated brake levers, which may or may not have brake cutouts in them to kill the motor while braking. It can be a cheap, simple, and easy upgrade. A full hydraulic upgrade can be much more expensive and involved, but the end result is more significant.
