Reciprocal space in the context of x-ray diffraction and a crystal lattice?
The unit cell space and reciprocal space are fourier transforms of each other. The unit cell indicates the stacking space between crystal elements. The reciprocal space is a similar kind of vectorial representation of the diffracted X-rays. The following reference explains it quite well, although it is not a simple concept. Unless you are smarter than me, you may have to read it 2 or 3 times to 'get it'. 

Reciprocal Lattices and Diffraction