What are floating/fixed calipers?
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I was gonna answer your question but something got in the way.
Anyways, here's the answer:
There is floating caliper and fixed caliper setup.
Floating caliper has the piston on the outside of the rotor only, but when you apply the brakes, the caliper squeezes the rotor from the outside, but also draws the inside brake pad also - there is a brake pad on the inside and outside of the rotor.
Fixed caliper has the same number of pistons on the inside and the outside, and basically the pistons would squeeze both pads onto the rotors.
The floating caliper design is more common because it's cheaper and effective (but not more effective than a fixed caliper setup). The floating caliper design is always used in single-piston applications, whereas fixed calipers are always used in multi-piston applications. Porsche uses fixed calipers, as does Mercedes but not in all of their vehicles. BMW uses single-piston calipers but I cannot confirm they used the floating caliper system, although this is probably the case. Fortunately, the new M6 will be BMW's first car to feature two-piston calipers!!
There is more detail to what I explained but I figured you're more interested in the summary, and it's hard to describe it without pictures.
Anyways, here's the answer:
There is floating caliper and fixed caliper setup.
Floating caliper has the piston on the outside of the rotor only, but when you apply the brakes, the caliper squeezes the rotor from the outside, but also draws the inside brake pad also - there is a brake pad on the inside and outside of the rotor.
Fixed caliper has the same number of pistons on the inside and the outside, and basically the pistons would squeeze both pads onto the rotors.
The floating caliper design is more common because it's cheaper and effective (but not more effective than a fixed caliper setup). The floating caliper design is always used in single-piston applications, whereas fixed calipers are always used in multi-piston applications. Porsche uses fixed calipers, as does Mercedes but not in all of their vehicles. BMW uses single-piston calipers but I cannot confirm they used the floating caliper system, although this is probably the case. Fortunately, the new M6 will be BMW's first car to feature two-piston calipers!!
There is more detail to what I explained but I figured you're more interested in the summary, and it's hard to describe it without pictures.
Most of what Emile said is true, but there are some technicalities. Floating calipers have the pistons on the inside of the rotor, not the outside. As the brakes are applied, the piston smashes the inner pad up against the inner surface of the rotor. As this pressure is applied, the caliper "floats" and slides on its mounting bolts in order to apply pressure on the outer side of the rotor with the fixed outer pad. Fixed calipers do not slide - they are rigidly attached to the spindle/hub/whatever. Fixed calipers have the potential to be more powerful than floating calipers because you can house more pistons in their housings. That translates to more braking pressure distributed more evenly across the brake pads. However, they are typically noisier and less reliable for the street - if the rotor becomes even slightly warped, the brakes will vibrate horrendously since the caliper is rigidly fixed to the suspension. A car with floating calipers is more tolerant to higher rotor runouts since the caliper can float back and forth on its mounting. Plus, as Emile stated, the floating caliper design is much cheaper to build.
Yes, the single piston calipers on BMW brakes are floating calipers. A single piston caliper has to be a floating caliper in order to apply braking force to both sides of the rotor. So, yes, the floating caliper is always used in single-piston applications. However, the floating caliper design can also be used in a two-piston application. In this case, both of the pistons are located on the inner side of the rotor. Basically it works identically to the single piston design described above, but instead of one huge piston, you have two smaller ones side by side. This is actually a very common setup - the previous bodystyle Mustang Cobras, the current Chevy Trailblazers/GMC Envoy family, I think the Chevy full size pickups, and many other cars and trucks use a floating 2 piston caliper. If the new M6 is going to have two piston calipers, they will almost certainly still be floating calipers.
Yes, the single piston calipers on BMW brakes are floating calipers. A single piston caliper has to be a floating caliper in order to apply braking force to both sides of the rotor. So, yes, the floating caliper is always used in single-piston applications. However, the floating caliper design can also be used in a two-piston application. In this case, both of the pistons are located on the inner side of the rotor. Basically it works identically to the single piston design described above, but instead of one huge piston, you have two smaller ones side by side. This is actually a very common setup - the previous bodystyle Mustang Cobras, the current Chevy Trailblazers/GMC Envoy family, I think the Chevy full size pickups, and many other cars and trucks use a floating 2 piston caliper. If the new M6 is going to have two piston calipers, they will almost certainly still be floating calipers.