A powder diffractometer does not have 3 axes to align the sample but only one. So how are the crystallites aligned to fulfil the Bragg conditions? As a matter of fact, they are not aligned, at least not on purpose. In a powder there are millions of crystallites that are randomly oriented. There will always be some that fulfil the Bragg conditions as long as the 2θ angle corresponds to a glancing angle of some lattice plane spacing d. This has the advantage that a powder diffractometer is of much more simple design than a single crystal diffractometer. However, a major part of information is lost: in single crystal diffractions the diffraction peaks occupy a three dimensional space which is called the reciprocal space. In powder diffraction we lose any information on direction and all peaks are projected onto a single line.
In other words: a powder diffractogram is a one-dimensional projection of a single crystal diffractogram. To get an idea just remember that a photograph is a 2-dimensional projection of 3-dimensional space. The following pictures should help illustrate this to you: