A set of planes gives a single diffraction spot at a particular value of 2θ. The picture here shows 2 spots as we have chosen a crystal that contains a mirror plane aligned so that two sets of planes fulfill the diffraction condition.

These 3 large crystals have slightly different orientations, so give diffraction spots at a few different positions.

As the number of crystals are increased the spread of spots increases and more values of 2θ can be accessed.

A powder has crystals in all possible orientations

As there are all orientations of the crystal, rings of different 2θ values are formed.