Geology: facies v. formations
A formation is a unit of stratigraphy. It refers to a set of sedimentary layers which are somehow distinct from those above and below them in the stratigraphic sequence. This may be because they have a different lithology, different sedimentary structures, or different fossil assemblages, or something else entirely.

A sedimentary facies is a set of rocks which where deposited in the same sedimentary environment. Typically deposition in this environment will leave behind distinctive signatures in the rock which can help to identify the environment of deposition.

Within a formation the facies can change both vertically and laterally. Equally, the facies may not change at all over several formations.