What the hell is eisenstein crteria?

I learnt this recently and for those who might really be wondering what eisenstein(not einstein) criteria is here is the mathematical form.
p.s. for those who find school maths difficult dont bother trying to understand.
(ie. Anson dont read on)

let f(x)=ax^n+bx^(n-1)+cx^(n-2)+...+yx+z. where a,b,c,...,y,z are constants.
let there be a prime p which doesnt divide 'a' but divides the other constants.
then if z is not divisible by p^2, the whole expression cannot be factorised.

most of you probably wont understand how this might be useful in maths but just to give an example, determine if there exist any intergral solutions for the expression
x^4+3x^2+6^x+6=0
now notice that the coefficients are 1,0,3,6,6
that thus the 'a' is not divisible by the prime 3 but the rest are.
thus in one look, one can say that the whole polynomial is not factorisable since
6 is not divisible by 9.
isnt that easy??
for those of you who read on without heeding my advise its ok you really dont need to know this for any tests!