Blow back is the simplest, as it does not lock the bolt to the barrel or receiver. It is held in place at the moment of firing by the weight of the bolt and the recoil spring alone. It is considered the heaviest action, as the weight of the bolt has to be enough to absorb the cartridge pressure.
Gas operation has a small hole in the barrel, allowing gas to either push on a piston, or in the case of the M-16, directly pushes on the bolt carrier, to which the bolt cam rotates the bolt head, allowing the bolt and bolt carrier to recoil rearward and cycle the action. Gas operation allows a lighter bolt and receiver, generally.
Delayed blowback does not actually lock the bolt to the barrel, but uses rollers which are forced into recesses around the bolt, holding the bolt stationary for a moment, and after the pressure from the cartridge starts to dissipate, the rollers come out of the recesses and allows the bolt to cycle. Delayed blowback is a bit lighter than a gas operated action, but not so heavy as a blowback type.
