ClassicPress started life as a hard-fork of WordPress 4.9; one of the differentiators is the Gutenberg block editor which started by replacing the TinyMCe editor (now generally referred to as the “classic editor”) and is now beig rolled out through other part of a WordPress site.
However, this is just one of the differentiators. One of the largest other differences is the decision making process used by ClassicPress is petition based with the community casting votes instead of a small number of contributors, or even one man, making the decision.
Viktor Nagornyy has a very good write up of the petition process. If you’re a member of the ClassicPress community, or a WordPress user looking for an alternative where you can make a difference, check out the petitions available for voting upon.