Hadron calorimeters are located outside the electromagnetic calorimeters. A hadron calorimeter measures the energy of hadrons, particles that are built up out of quarks.
A hadron calorimeter is constructed in the same way as an electromagnetic calorimeter, but iron is often used instead of lead.
Hadrons pass through the EM calorimeters relatively unaffected so that their energy can be measured in the hadron calorimeter. The energy is registered in a similar way as in electromagnetic calorimeters, but here it is the strong interaction that creates the secondary particles. Another difference is that the hadrons travel much further before they are absorbed in the calorimeter. Therefore hadron calorimeters are much thicker than EM calorimeters.