Moodlet

Moodlets are reflections of mood altering events or situations that a Sim has done or experienced in The Sims 3 and The Sims 4. Moodlets may be positive, negative, or neutral. Positive moodlets will increase a Sim's mood, while negative moodlets will decrease it. Moodlets will also affect a Sim's emotions. Which moodlets appear and when may depend on a variety of factors, including the Sim's age, traits, and life state.

Moodlets may also have secondary effects, such as improving skill gain, making it easier to burn to death, or making a Sim fatter when they eat, but the benefits are not always listed on the moodlet itself.

Some moodlets happen for an indefinite amount of time while others are timed. Some moodlets lead directly from one to the next, such as "Strained" leading to "Stressed Out", or "Sleepy" leading to "Tired" then "Exhausted", but all 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!". Moodlets that do not specify a time end when certain events occur, such as leaving the area that gives the moodlet.

Clicking on negative or neutral moodlets will queue the appropriate interaction to resolve the issue. 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 occasionally backfire on the Sim, creating or exacerbating negative moodlets instead of removing them. Moodlets can also be removed when the testing cheats are enabled by ctrl-clicking on them. This can cause glitches with the game if certain moodlets are removed. For example, if the "Baby is Coming" moodlet is removed using this method, the pregnancy will be unable to come to term and the Sim will be unable to age up.

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.