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 often that Sim will 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) all 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 there 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.
How free will works
Unlike the the Sims free will (in which a sim would happily starve to death because the TV was closer without the guidance of a patient player), 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.
Random autonomous reactions
Some sims have special actions which they can preform at random. These include a Vampires's 'bleh' and a sloppy sims need to 'pass wind'.