Ordinary and Singular Points in Power series

[susi]

hi,
what is the difference b/w ordinary and singular points and what is the behaviour of Analytical functions on these points?...
please tell me in brief with examples.
thanks!

[XMR]

Function f is analytic in point z' if it has derivative (in complex meening) around z' point.
f(z)=U(x)+iV(y);
z=x+iy
If this condition isn't satisfied the function(for example power serie) isn't analytic in this point.

Derivative (complex) exist when ∂u/∂x=∂v/∂y and ∂u/∂y=-∂v/∂x