Critical results are an optional Fudge rule for GMs who like the idea. A natural rolled result of +4 can be considered a critical success the character has done exceptionally well, and the GM may grant some special bonus to the action. Likewise, a natural result of -4 is a critical failure, and the character has done as poorly as he possibly can in the given situation.
Note that achieving +/-4 with die modifiers does not count as a critical result, though the character has done exceptionally well or poorly. When a natural critical result is rolled, the GM may ignore what the rolled degree would be, and treat it as an automatic beyond Superb or below Terrible result.
Optionally, if a character gets a rolled degree four or more levels better than the Difficulty Level, he has gotten a critical success. Likewise, four levels below a Difficulty Level is a critical failure.
A critical result in combat can mean many things: one fighter falls down, or drops his weapon, or is hurt extra badly, or is stunned for a round and can’t even defend himself, or is temporarily blinded, or knocked out, etc.
The GM should be creative, but not kill a character outright.
The GM may even wish to make a table, such as these sample melee critical results:
2 Blinded for the next combat round no defense or offense!
3 Fall down: skill at -2 for one round.
4 Armor badly damaged no armor value rest of fight!
5 Weapon finds chink in armor do not subtract for armor.
6 Off balance skill at -1 next turn.
7 Drop Weapon.
8 Weapon breaks, but still useful: -1 to damage.
9 . . .
And so on finish and customize to your tastes.
This is an easy way to achieve a lot of detail without complicating Fudge. Those with Internet access are invited to add any interesting critical results tables they create to the Fudge sites.