NeedsIf you played The Sims, then you should be familiar with the Needs system. These are, as the name implies, the basic needs of your Sim; if you want your Sims to succeed in their personal and professional lives, you're going to need to ensure that their Needs are taken care of.
There are eight Needs in total: Food, Comfort, Hygiene, Bladder, Energy, Fun, Social, and Environment (formerly Room). Your Sims have a different score in each Need; taken together, their overall Need fulfillment will be combined to form their Mood, which is something of an overall barometer of happiness for the Sim. You will need to constantly monitor these Needs as you progress through your Sim's daily life, as an unhappy Sim will be less likely to successfully socialize and will perform poorly at work or school.
Each Need will constantly degrade over time, no matter what actions your Sim performs, although some activities will hasten the decay of specific motives. Using the bathroom or exercising, for instance, will have more of an impact on your Hygiene Need than will watching television. The goal isn't to keep each Need constantly maxed out, though; this is actually impossible, and attempting to do so will prevent your Sim from going to work or achieving their Wants. A more feasible goal is to keep every Need above two-thirds or so, which should keep them happy enough to obey your commands and be successful in most of their interactions with other Sims. Needs that are extremely low have a disproportionately large affect on your Mood, so it's usually best to concentrate on the lowest Needs first.
Luckily, in Sims 2, the AI for your Sims has been improved over that found in the first Sims game. Now, if your Sims are left to their own devices, they'll automatically attempt to work on their most pressing Need, rather than choose one at random. This will let you let the AI control your Sims a bit more efficiently than they did in the first game; you'll still want to guide them as often as possible, of course, but you'll be able to let them go a bit longer without direct control if you want to concentrate on another Sim.
BladderBladder is the simplest Need to take care of: simply steer your Sim towards a toilet, let him or her use it for a few minutes, and you'll be taken care of. This refills completely when you manage to make it to a toilet, and doesn't take long to completely max out, no matter what kind of toilet you use, so feel free to let your Bladder Need drop fairly low before you bother to deal with it. Just don't forget about it, or prevent your Sim from reaching a bathroom in time, as a completely depleted Bladder will result in a messy situation, and one seriously unhygienic and unhappy Sim.
In addition to degrading naturally over time, Bladder will also be degraded when your Sim eats. Relieving the Bladder will cause your Hygiene score to decrease, so you might want to save your bath or shower until after you hit the toilet. Some of the nicer toilets will also give your Sim an appreciable Comfort boost while they relieve themselves; Comfort can be difficult to come across outside of sleeping, so these extravagant commodes are definitely worth your money.
Sims that gravitate towards the Shy end of the Shy/Outgoing trait line will refuse to go to the bathroom if there's another Sim in the same room as the toilet, and will in fact throw a fit, wasting precious time out of everyone's day. For this reason, it's always best to enclose your toilet in a small space, with a sink being the only other required item. If you intend to throw parties, two bathrooms are essentially a requisite; these should ideally be placed near the main socialization area.
Note that your Sloppy Sims will gain a small amount of Bladder replenishment when they take a shower. Just don't tell their housemates.