An identity permutation is an even permutation as (1)=(1 2)(1 2).
The composition of two even permutations is again an even permutation, and so is the inverse of an even permutation: the even permutations of n letters form a group, the alternating group on n letters, denoted by An. This is a subgroup of the symmetric group Sn and contains n/2 permutations.
An odd permutation is a permutation which is not an even permutation, equivalently, it is a product by odd number of transpositions.
See fifteen puzzle for a classic application.