Topological orders describe novel states of matter that cannot be described in terms of symmetry and local order parameters. They feature degenerate, topologically protected ground states that are robust agains arbitrary perturbations. For this reason, they are excellent candidates for building a quantum computer. Moreover, they possess a non trivial pattern of long range entanglement that makes them unique from usual states of the matter: they have a topological entanglement entropy. In this talk, I will introduce the concepts of topological order, topological entropy and quantum memory and show the profound interconnection between these concepts. In particular, we will discuss the problem of topological order and quantum memory at finite temperature and show that there is a unique mechanism that protects the memory and the topological entanglement.