Also known as the rising factorial, it is given by:
(n) &Gamma(a+n)
(a) = a = a(a+1)(a+2)...(a+n-1) = -----
n &Gamma(a)
The Pochhammer symbol appears in series expansions of hypergeometric functions, and is often used in combinatorics as well.
It also plays an important role in the calculus of finite differences, as the forward difference of (x)n is simply n(x)(n-1), making linear combinations of expressions involving Pochhammer symbols analogous to polynomials in the differential calculus. Powers of x can be turned into equivalent expressions involving Pochhammer symbols using Stirling numbers. | 
