SkillsThere are seven main skills that your Sim can attempt to learn in Sims 2: Cooking, Mechanical, Charisma, Body, Logic, Creativity, and Cleaning takes up the rear as a new inclusion. These are important to your Sim because of the fact that the careers in the game will require you to possess skills of a certain level before you can advance to the next promotion level, but they also have an impact on many other areas of your Sim's life.
So far as your careers are concerned though, each career will have three primary skills that it will emphasize. If you want to reach the highest level of promotion in a career (level 10), you'll almost always have to maximize at least one skill (also to level 10); the other two skills will either have to be maximized or raised fairly high themselves, so you will have to spend some serious time studying if you want to reach the pinnacle of your chosen field. (See the Careers section for specific skill requirements.)
CookingThe Cooking skill has gained newfound importance in the Sims 2. While it still helps prevent your Sim from starting fires (try to get up to Cooking level two before you go crazy at the stove), you're now able to learn new recipes as you proceed upwards and onwards with your Cooking skill. As always, the primary benefit of a high Cooking skill is that your Sim will be able to produce meals that will more quickly satisfy not only his own Hunger score, but also that of any other Sims that he cooks for. Having a Sim with a high Cooking score can thus eliminate the need for everyone else in your household to prepare their own food, if you manage to time their routines so that they all eat at roughly the same time. This will also help their relationships, as Sims naturally gravitate towards the same seating area and talk when they eat concurrently.
To raise your Cooking skill, you can obviously just prepare food over and over again, but this isn't very practical, just because you'll create huge messes of uneaten food and/or reduce everyone else's Fitness score by stuffing them with food that they don't need, and that's not even mentioning the fact that you actually have to buy food to stock your fridge with now. A better choice, especially at low levels, is to select a bookshelf and choose to Study cooking, but you can also watch cooking shows on television or, if you've advanced far enough into the Culinary career track, use the Chocolate Manufacturing Facility to create salable chocolates. This last choice is likely the best, as you'll gain your precious skill points while creating a bit of income for your household.
MechanicalMechanical isn't the most useful of skills, outside of using it for job requirements. In everyday life, a Sim can use her Mechanical skill to repair broken appliances (which also raises the skill), but this comes with risks: if you manage to seriously bungle a repair job on an electrical item, for instance, your Sim may be electrocuted, which will screw around with your Need levels quite a bit, and may even result in the loss of a skill point, or even instant death!
In point of fact, though, most quality items won't break down that often, so you shouldn't bother training up Mechanical unless you need it for a career promotion qualification. (If something does break, you can always call a Mechanic on the phone if you don't want to bother repairing it yourself.) If that's the case, though, you can study at a bookshelf, try to tune a piano, or use the Surgical Training Station that's a reward for moving up the ladder in the Medical career.