The Ewald sum calculates coulombic interactions for periodic boundaries and is like using an infinite cutoff for point charges. The way it works is that the long-range portion is calculated in reciprocal space using a Fourier series and the short range part is calculated directly. In reality it is a little more complicated: damped short range calculations (including self-interactions) are also included in the reciprocal space part and have to be subtracted off.
Stan