Fury

Fury or jealousy is a relationship feature that was added in Nightlife. If a Sim is upset with another, he or she may gain a fury mark on the relationship panel, shown by a red haze over a Sim's portrait. It decreases the relationship drastically while it's in effect, putting both Daily and Lifetime scores below -50, depending on the 'crime' committed. Fury is also featured in The Sims: Life Stories, The Sims: Pet Stories and The Sims: Castaway Stories. Fury in The Sims Stories has the same effects as in The Sims 2.

Causes of Fury
Fury can be caused by multiple things.

Fighting
Doing certain annoying interactions like "Throw Drink" will cause it, as do all "Fight" options. Sometimes, water balloon pranks cause fury, but it may not have been intended to be that way, as even a Sim who laughed and pranked back may get furious. Supposedly, it has something to do with cleaning up the puddle.

Affairs
A surefire way to make a Sim see red is to be caught in an affair. It's a double-fury, with the Sim angry at the cheater and the one he or she was with (if they have never met, they meet at that point, usually starting at a healthy -100/-100 in relationships).

Robbery
One last way to cause fury is by getting robbed. Although it is completely random with the exception of cheats, the Sim will be angry at the burglar for a long time.

Duration
The length of fury depends on the offense. Simply throwing a drink may only last a few days, if that long. Getting caught cheating, however, ensures the Sim will be upset at both parties for a very long time, possibly even after fury wears off. Surprisingly, it seems burglary lasts the longest unprovoked, but again is not completely reliable. When Fury ends and all the red on the panel clears up, there is a bit of a relationship boost in addition to the normal decay to 0/0 scores. This is sometimes used for Sims that were already at rock bottom to gain some points.

Blind spot
If you only have The Sims 2 and none of the other expansion packs after 'Nightlife' (Which has the 'Fury' thing), then there will be no fury, so say if you have one Sim catch their husband cheating on them, and they have 100/100 relationship score, which after they see the other Sim cheating makes it a -100/100 score. If they then divorce them, of course the Sim starts  walking off the property. This is where you leave the household, save what happens, move them out, and move them back again. The relationship score between the two divorced Sims will be about 26/100.

Unlocked Interactions
A furious Sim often gets a thought bubble of the one he or she is angry with, with flames coming off it, and may fight with the other Sim more often than usual. They also get the option to "Bad Mouth" about the fury recipient to someone else, although this causes a relationship decrease for those two. If the furious Sim does not live on the lot, they may sneak to the home at night, usually to either steal newspapers, steal lawn gnomes, or kick over trashcans. Furthermore, it's possible that a ghost that died furious will be extra angry in the afterlife. At the very least, it seems they still think about the one they hate. Furious Sims will often want to see the ghost of the Sim they are furious at.

Apologizing
The Sim that did the 'crime' can try to repair the relationship by using the "Apologize" interaction.