Skill Challenges are compared to a Challenge Rating, which roughly measures how difficult they are.
Routine: A task so easy, you barely notice performing it. Even rank amateurs and raw recruits usually succeed at Routine tasks. Ex.:
Easy: A relatively simple task, something amateurs find too complex, and entry-level workers find challenging, but competent professionals almost always succeed at. Ex.: Taking off or landing an airplane in clear weather. Diagnosing a common disease. Swimming a mile.
Moderate: This sort of task is the bread-and-butter of veterans (who succeed most of the time), but the untried and inexperienced find them daunting. Ex.: A reporter writing a newspaper column or story.
Difficult: Veterans usually succeed at these sorts of tasks, and standout members of a profession nearly always succeed, but entry level employees usually fail. Ex.:
Formidable: Something seasoned characters struggle to achieve, but luminaries usually succeed at. Ex.:
Grueling: A task one of the best in the world fail at, more often than not. Ex.:
Monumental: Tasks the foremost expert in a field fails at most times. Ex.:
Nearly Impossible: Even a DaVinci or Napoleon finds these tasks difficult, failing more than half the time. Ex.:
[Note: Obviously, examples are missing for most of the CR ratings. As skill descriptions are written up, these examples will be fleshed out.]