Autonomous reaction
An autonomous reaction is one which a Sim does that isn't player defined. These can be reactions to the environment, for example, a smelly rubbish bin, an unusual event, for example, a Sim dying, a bad memory, a missing object or simply something for a Sim to do if you haven't told them to do anything - providing free will is on. This adds an extra dimension to the game, making the Sims unpredictable and somewhat free from complete player domination - however with most actions a Sim will terminate the action if either the player tells them to do so or the player tells them to do something else.
Memory reactions
If a Sim suffers from a bad memory, that Sim will often spend a few days randomly stopping actions they decided - or were told - to do, and crying or sighing while thinking about the memory. There is nothing in-game the player can do about this.
Event reactions
These include reactions to fire and birthdays, weddings, births, deaths etc. When such an event takes place all the Sims on the lot are called to the place where the event is taking place. With the exception of fire (but only because the action pops up again immediately after - most annoying) these actions can be canceled. Even if a Sim does not witness an event such as dying, they will still react to it when they return from where they are or you visit their lot etc.
Environment reactions
These include the 'attraction' and 'smells yummy' actions that appear when a Sim walks into a room where there is a Sim with similar chemistry or a plate of well-cooked warm food or a similar scenario. Sometimes they are nothing more than a thought bubble and a facial expression.
In The Sims 3, A grubby environment will result in a negative Moodlet. As the environment's condition worsens, the moodlets are more and more negative.
How free will works
Unlike free will in The Sims (in which a Sim would happily starve to death without the guidance of a patient player, because the TV was closer), The Sims 2 has a more sophisticated system (though not perfect) way of giving Sims some common sense. It works as a scoring system. Each object on the lot gives out a signal to Sims near by, with the fridge giving out a larger signal than the TV or other less vital object. If a Sim has a low enough hunger score (i.e. is hungry) the fridge's signal is more likely to acquire a response from the Sim (according to the primary guide) basically what a Sim does is determined by needs and closeness instead of just closeness like the first Sims game.
The 'signal strength' is based on how much the object can satisfy a particular need, the Sim's personality, their current need levels, and the objects' distance strength. Objects broadcast a signal saying how well they can satisfy a particular need. Two chairs for example, one cheap and one expensive, will broadcast different strengths for how well they can satisfy the comfort need. The expensive one will win in this case, unless it's already taken by another Sim. Some objects can 'cheat' and broadcast need fulfillment that they don't give, such as a ringing phone.
While a Sim's personality doesn't affect their choices as much as the object's signal strength themselves, it can help decide what a Sim would choose. For example, a serious Sim is more likely to choose a book or the chess table over the TV. An active Sim is more likely to choose a physical activity, etc.
The third modifier to signal strength is the Sim's current need levels. If a Sim has a low hunger score, going to the fridge is going to be a high priority. Same goes for things like comfort, energy, bladder, etc. Often, this can be one of the larger modifiers to the signal strength.
The last modifier is the signal's distance. Some objects broadcast over the entire lot (beds), while others only over a short distance (newspaper). Each object is different, and it can help the Sim decide which object to choose. Why would a Sim want to go to the fancy toilet on the other side of the lot when they can just use the (mostly) clean one in the next room?
Random autonomous reactions
Some Sims have special actions which they can perform at random. These include a vampire's 'bleh' and a sloppy Sim's need to fart.