Sims have 8 different needs to function well. These are also known as motives.
This is a very important need. When a Sim's Hunger meter is completely empty, they will die. The Hunger need can be fulfilled by eating. Things that can be eaten are meals, quick meals, snacks, or pizza. The higher someone's cooking skill, the more satisfying the meals.
Comfort, while not as important as Hunger, can bring down a Sim's overall mood if not tended to. Basically this need can be fulfilled by sitting on chairs or sleeping in a nice, comfy bed. Baths also satisfy the comfort need, so you can kill two birds with one stone by using a bath instead of a shower.
This can be a rather annoying motive, as failure to attend to it with due speed can result in another need to drop rapidly (Hygiene). The bladder need is satisfied by using a toilet, or urinating on the floor when the Bladder meter is empty.
Hygiene is basically a Sim's personal cleanliness. This need is satisfied by making your Sim brush his/her teeth, wash hands or having a shower or bath. Having baths carry the additional bonus of satisfying the comfort need at the same time.
All work and no play makes Jack a dull boy. The same can be said for Sims. When they haven't done anything they find entertaining for a while, they refuse to do anything that improves their skills, as work is dull. The Fun meter is filled by a large variety of things, from books to computers to toys to televisions.
The Social need is a Sim's need to communicate and be with other Sims. When this need isn't satisfied, their mood can take a dramatic downturn and become depressed. This need can be fulfilled by making your Sim do anything with another Sim, whether it be talking, eating or dancing with each other, among other things.
This motive is essentially what a particular Sim thinks of the room s/he is in. Dirty rooms with no wallpaper or flooring will send the Room meter close to the bottom. Dirty rooms can send this even lower. Sims who aren't very neat won't mind the mess as much.
Without Energy, Sims won't do much. They'll be grumpy and will fall asleep on the ground, where their comfort level will go down faster. This problem can be averted if the Sim has a nice bed and you send them to it. They can also drink coffee to send up their Energy need a little bit, but this will negatively affect their Bladder motive. Sims can also nap on a lounge, or (as stated earlier) the floor.