For this disucussion, Joe is flat average stats for a human. He is competent in whatever it is you're trying, but not to elite nor professional (IE, Master's Level or equivalent) levels.
For Level based games, usually assume level 1.
Exemplars for Illustrative Purposes
I refer to the following, so the precis is presented so you can follow the actual answer portion without doing research...
In MegaTraveller, the task system is based upon a roll of 7+ on 2d6 being "Routine" for a person with Skill 1 and +1 from attribute. MT also defines skill 1 as employable, and Skill 3 professional (as in, doctoral or masters degree trained fields should result in skill 3).
which means it's a mishap on a 2-3 (3/36 chance), fail on a 4 (also 3/36), leaving 5 a marginal (4/36), 6 (5/36) moderate, and 7+ (21/36) exceptional success.
MT also defines Impossible as 19+... given that there's effectively a DM+4 for taking extra time (but it's expressed as a difficulty shift), and given the DM+1 from stat, and DM+1 from skill, Joe Normal simply can not perform an impossible task. His max roll of 12, +4 for extra time, +1 for stat, +1 for skill is a mere 16. Even with peak stat, for DM+3 instead of +1, without a skill better than 1, Impossible remains just that: you can't get there.
A natural 2 is always a failure.
The overall scale, and Joe Normal success rates (Adj TN, chance of Adj TN, and % chance):
Simple 3+ (3+ 35/36 97.3%)
Routine 7+ (5+ 30/36 83.3%, ExTime 3+ 35/36 97.3%)
Difficult 11+ (9+ 10/36 27.7%, ExTime 5+ 30/36 83%)
Formidable 15+ (13+ 0/36 0%, ExTime 9+ 10/36 27.7%)
Impossible 19+ (17+ 0/36 0%, ExTime 13+ 0/36 0%)
The labels and processes are the same, but the definitions of Joe normal differ. Joe has a DM+2 from attribute, DM+1 from skill, and the die-roll is 1d10, not 2d6... so the odds look like:
Simple 3+ (2+ 90%) note minimum failure of 10%
Routine 7+ (4+ 70%, ExTime 2+ 90%)
Difficult 11+ (8+ 30%, ExTime 4+ 70%)
Formidable 15+ (12+ 0%, ExTime 8+ 20%)
Impossible 19+ (16+ 0%, ExTime 12+ 0%)
Twilight 2000 2E, Dark Consipiracy, and TNE
1d20 for Stat+skill or less for a difficult task. Average stat is 6, baseline skill is 1. Same labels, extra time rules, and many other similarities with MT and 2300AD... including publisher and design staff. Each level is a double or half, extra time is a full level.
For Joe Normal:
Simple x4 (19- 95%)
Routine x2 (14- 70%)
Difficult x1 (7- 50%)
Formidable x1/2 (3- 15%)
Impossible x1/4 (1- 5%)
WFRP 2E, Dark Heresy, Rogue Trader, and Deathwatch
The default task level, called challenging, for a skilled charcter is a percentile equal to attribute; this means an default level roll is roughly 31%, tho' the overall range is 17% to 70% for humans... But Routine is defined as +20 to the percentage. Trivial is +60, Difficult is -10, and Hellish is -60. Which means a hellish task is bloody hard for even the best characters, with 70% stat, +20 from skill, and +10 from a relevant trait; with +10 from help, peak chance is 50% for maxed out experience uberman.
D20 and 3.X D&D
I'm not positive I've got the labels right, but given the DC's, Joe Normal, level 1:
Automatic: DC5 (95% or Take 10 for 100%)
Easy: DC10 (70% or Take-10 for 100%)
Moderate: DC15 (50%)
Difficult: DC20 (20%)
Very Difficult: DC30 (1% if using the open ending option)
Note that the Take-10 mechanic means that Easy Tasks are done automatically if skilled in them.
Generally, most systems take a baseline skill level (and if relevant, attribute level), and define task labels relative to this. They don't really give hard and fast ideas, but generally put the chances of success for a trained character at a routine task about 50-75%.
All the other breakpoints vary widely, but in general, most will have a set minimum success chance, and most also have a maximum success chance.
A few games instead label difficulties by how good you have to be to have a given chance.
The label order isn't even terribly consistent, other than the basic 5: Automatic, simple, easy, moderate, difficult. Exactly what those mean, however, varies too widely to generalize.
It truly means, no matter what system, you need to work out the percentages yourself, and you need to figure out what a particular difficulty label truly means. It also helps to figure out what the defined baseline is for those labels.
Note the extra time listings for MegaTraveller; when not rushed, a competent character can take extra time and make routine tasks fail only 2.7% of the time; even semi-skilled characters (skill level 0 but not unskilled) doing so is still that 97.3% chance of success.
Note the similar break points despite the different die-roll for 2300AD and MT... same company, similar meanings. Actually, the task system for MT was invented for use with the prior Traveller edition, by a 3rd party (Digest Group); It was then adopted and adapted for 2300AD, THEN reverted back for MegaTraveller, since MT was essentially collated by Digest Group. T2K2.2 and Dark Conspiracy have similar breakpoints... but much wider difference between peak characters and poor.
It's all just guidelines. Shortcuts to get you up to a better level of GMing competence faster, by giving you labels. But those labels have to mesh well with the skill levels listed, too. When they don't, when the odds are too out there for players, they often reject the whole system because the labels don't meet their expectations. With MT, I've lost a few players because they thought Routine was too hard as written, and no level should be labeled "Impossible"...
The Usual Approach
The usual approach is this: The labels usually work for the "Normal Guy" with "A reasonable level of skill" doing a "Normal task"...
You just need to approach all task labels from the idea of "How hard would Joe Normal find the action" and then pick the difficulty.
As a designer, the folks at GDW followed this same approach.
The Alternate Approach
A few games define difficulties by skill level at which success is expected. Fudge, as well as Fate and other such derivatives of Fudge, defines skill levels and difficulties with the same "ladder" of skill. Just define what skill level you would expect to succeed 60% of the time, pick it, and there's the difficulty.