Play by email is a slow (though not as slow as the old play-by-mail!) method of roleplaying. The GM sends an email out to all the players describing the situation. The players respond by writing what they are going to do (often times including conditional actions). Once the GM has all the responses (or the time limit is up), the GM resolves the consequences of those actions, then decides what's going to happen next, sends out the report to the other players and waits for their next reactions, etc. For Champions I would imagine each mail-out would represent a full turn (though I've never participated in a RM PBEM).

The upside is the story tends to be more roleplaying and less number crunching, you can take more time to consider what you want to do, and in the long run some good stories end up emerging. The downside is it can seem like it takes forever, with one turn occurring every two days (if you're lucky).

Definitely not for the impatient type.