Moodlet

Moodlets are reflections of mood altering events or situations that a Sim has done or experienced. They may appear from interactions they experience, surrounding encounters or may even spontaneously appear from their traits. These moodlets may be positive, negative, or neutral. A moodlet that is positive or negative respectively increase and decrease the Sim or pets mood bar. The Sim's mood will be determined by the overall calculated score from each moodlet that have been accumulated during host's life. Certain moodlets may be exclusive to the Sim's age, life state or trait.

Some moodlets happen for an indefinite amount of time while others are timed. Moodlets that do not specify a time, end when certain events occur, such as leaving the area that gives the moodlet.

Some timed moodlets transition directly from one to the next, which are common with those related of a motive or an event leading to an outcome (such as impeding birth or death) unless interfered. These have the purpose of alerting the player of incoming misfortunes or expectancy, getting them to take actions or be prepared from upcoming happenings.

Many other moodlets are not mutually exclusive and so a Sim can have "Vile Surroundings" and "Beautifully Decorated" at the same time, or multiple instances of "It's a Boy!" or "It's a Girl!". In The Sims 3, If when a need is low enough or high enough, its respective moodlet of the corresponding effect type will appear nearby the motive's bar. In The Sims 4, a moodlet will influence a Sim's emotions. Moodlets that do not specify a time, end when certain events occur, such as leaving the area that gives the moodlet.

A moodlet aside from determining the mood may also have secondary effects. These are (but not limited to) such as improving skill gain, alter fitness, inflict particular behavior, enforce an interaction and so on. However, these benefits or penalties are not always mentioned on the moodlet itself.

Clicking on negative or neutral moodlets will queue up an interaction that remedies it. For example, clicking the smelly moodlet will make the Sim take a bath or shower, clicking the sleepy moodlet will make the Sim go to sleep, etc.

Moodlets can be removed using the Moodlet Manager lifetime happiness reward. The manager is not perfect, and may affect the Sim with a red beam causing negative effects instead of removing them.

Moodlets can also be removed when the testingcheatsenabled on command is used by ctrl+clicking them, removing need-related moodlets may cause the need to be refilled. This can also cause a glitch in the game, when a pregnant Sim has the "baby is coming" moodlet and it is removed the baby will never be born and there is no option at the hospital to deliver the baby, also the Sim will not be able to age up, exercise or maybe even die.

When setting a lot's Beautiful Vista moodlet score in the Create a World tool for The Sims 3, the label says "Set Beautiful Vista Buff Score". This may hint that moodlets were going to be called buffs.

Buff
Buffs are focus altering effects because of situations or events that happened to the Sim in The Sims Medieval. They are very similar to moodlets in The Sims 3 and act the same despite the fact that they relate to a Sim's focus instead of their mood. Like moodlets, buffs can be positive, negative or neutral as well as lasting for either a limited amount of time or indefinitley, though this kind of buff ends when the event or situation that gives the buff ends (for example, leaving a room that gives the buff "Decorated" will make the buff disappear).

Some buffs lead to another ("Moderately Injured" leads to "Minor Injured") but all other buffs are not mutually exclusive, so Sims can have different buffs at the same time.

Some Fatal Flaws (and the Whale Ate My Parents trait) will give the Sim a negative buff randomly or at some interval, even if keeping high focus.