Case is the "thing" that "hops" between the different "stations" (activities) of a process (imagine the process like a pipe):
The object we want to track to see its path and transformations along a process.
Transaction is an atomic activity or transformation that is suffered by the case.
Transaction data usually is used to fill the activities Table in Celonis
Just my 0,02 €
HTH
Agreed with Guillermo. Also this Celonis blog post (link) has a helpful illustration on it "If we think about a process as a road trip, each process variant would be a potential route. Each activity within a process would be a waypoint along a route, and the connections between activities are like the roads that connect the stops. And, each trip a person makes along a particular route would be a case."
Thanks for your reply both Mr. Gost and Mr. Saffran. I got the gist of it :)