  The linear span P_n of the sums of all permutations in the symmetric group
S_n with a given set of peaks is a sub-algebra of the symmetric group algebra,
due to Nyman. This peak algebra is a left ideal of the descent algebra D_n; and
the direct sum P of all P_n is a Hopf sub-algebra of the direct sum D of all
D_n, dual to the Stembridge algebra of peak functions. In our self-contained
approach, peak counterparts of several results on the descent algebra are
established, including a simple combinatorial characterization of the algebra
P_n; an algebraic characterization of P_n based on the action on the
Poincar'e-Birkhoff-Witt basis of the free associative algebra; the display of
peak variants of the classical Lie idempotents; an Eulerian-type sub-algebra of
P_n; a description of the Jacobson radical of P_n and its nil-potency index, of
the principal indecomposable and irreducible P_n-modules, and of the Cartan
matrix of P_n. Furthermore, it is shown that the primitive Lie algebra of P is
free, and that P is its enveloping algebra.
