In the real world, a quantum system may be affected by the surrounding environment, and the overall effect is a loss of information and energy leading to decoherence. In this case we should describe the system as an open quantum system and its dynamics is no longer unitary. In this scenario, the quantum operation formalism allows to describe the evolution of the quantum system in a wide variety of circumstances. In the quantum computation context, in particular, it is useful to deal with the (quantum) errors that can occur during a computation. In this chapter we provide some physical and mathematical intuition of the decoherence process and introduce the main noisy maps, that may affect a qubit during the time evolution, namely, the bit flip, the phase flip and the bit-phase flip maps. We also describe other maps that are of general interest: the depolarizing channel, the amplitude damping channel and its generalized version, and the phase damping channel.