For one thing, the Snyder Cut is 214 minutes long, whereas the theatrical Justice League clocked in at exactly two hours.
Other notable features of the Snyder Cut include Darkseid appearing, General Swanwick being revealed as Martian Manhunter and expanded backstories for Flash, Cyborg and Aquaman, to name just a few.
While some director’s cuts of movies are only slightly different from the theatrical cuts, Justice League is definitely one of this instances where what was shown in theaters drastically differed from what the filmmaker originally envisioned.