Moodlet

Moodlets are reflections of mood altering events or situations that a Sim has done or experienced. These moodlets may be positive, negative, or neutral. 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.

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, or to drown when swimming, but the benefits are not always listed on the Moodlet itself.

Clicking on negative or neutral moodlets will add the appropriate action to the bar. 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. In The Sims 4, a moodlet will influence a Sim's emotions.

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.

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.