Brake calipers are the units which hold the brake blocks and force them towards the wheel rim to slow the bike down. The blocks sit at the end of pivoted ‘arms’ which when stretched by cable move inwards and release outwards again when the cable is released.
There are several kinds of brake calipers:
Single pivot - both arms of the caliper move around one point
Dual pivot – both arms of the caliper are individually pivoted giving a much more linear and improved braking action (most road brake calipers are this type these days)
Cantilever – these are the longer separate arms seen on cyclo cross and touring bikes which each sit on individual bosses and the cable pulls the arms towards each other, the advantage being they give more mud clearance and you can use thicker tyres without brake rub
TT Centre Pull – these are a derivation of the above cantilever but they have a much lower profile for fitment to the rear of time trial or aero road bikes
Disc caliper – with disc brakes becoming more common on cyclo cross, touring and even road bikes, a disc brake caliper is a mechanism that houses the brake pads which push against the disc to slow the bike down (same as a car or motor bike disc brake does). These calipers aren’t in their traditional place on the frame and fork; they sit much lower on the bike at the bottom of the fork and rear of the frame close to the hubs where the discs are attached to. Nearly all road bike discs at the moment are cable operated so if you want to use hydraulic calipers you will need to fit an adapter to use them with cable brake levers.
Each manufacturer has their own unique variation of above but all follow the same format of single pivot, dual pivot, cantilever or disc. Some frames with mudguard clearance require a ‘long drop’ caliper as the brake mountings are further away to allow for extra space taken up by mudguards.
Brake calipers should be matched with brake levers from the same manufacturer as some pull ratios are slightly different, especially on newer models. Please note that Shimano and SRAM brake calipers have a quick release mechanism, this small lever moves the arms further outwards without the need to loosen the cable to ease wheel change, built into the caliper but Campagnolo has the quick release action built into the brake lever so if you mix Shimano and SRAM levers with Campagnolo calipers you will not have a quick release option.