Each round, the player plays a random map and must pick 2 Torment cards. Difficulty modifiers added by these cards will create a custom difficulty just for this run (as of Phasmophobia's v.0.7 "Apocalypse" update), which starts as Professional difficulty and gets harder as the run goes on.
The last card offered will always be a mystery, and all maps are small except during the 3rd and 5th round.
Failing to beat a map or to complete a challenge will fail the run.
Cards have a difficulty rating. As difficulty increases, easier cards become rarer up to the point of not being offered anymore, while harder cards become eligible to be offered and get more common. Card difficulty increases naturally as the run progresses.
Some cards are also naturally rarer than others, depending on how frustrating or too helpful they are, so that on average runs have a good balance. Furthermore, a few cards only make sense in a certain context, and thus won't be offered except in very specific circumstances.