Weight Watchers is basically counting calories with much simpler addition.
Instead of counting calories WW has come up with a formula that takes fat and fiber into account. In essence, encouraging you to eat more nutritionally rich foods.
A rough approximation is 1 point = 50 calories, so if you get 26 points per day that is a 1300 calorie a day diet. Except that you don't count any of the calories in "0 point foods" like most vegetables, so assuming you eat lots of veggies while dieting, you are looking at about 1500-1600 calories a day for a 200 lb person.
In addition to your points per day, you get 35 "extra" points or approximately 1750 "extra" calories per week. This allows you to eat extra on the days you are hungrier or go all out occasionally if you are hanging at the pizza joint with your buddies without feeling like you have blown your diet.
For me, WW has a few advantages over simply counting calories on my own.
1) WW has simplified counting points to where it is almost a no brainer. There are books where you can look up the points, little slide-rules that allow you to look up points based on the nutrition label, and websites like 3FC that have a
Point Counter that does it for you. Also, since most of us eat the same foods, after you do it for awhile, counting points is easy. I rarely need to look up how many points are in the foods I eat. I know an apple is (1), a bowl of oatmeal is (2) and 3 oz of grilled lean meat is (3).
2) Having someone weigh me each week and write it down in a book holds me accountable. Knowing Monday is weigh in makes me more likely to exercise over the weekend and more likely to eat better over the weekend.
3) Sitting in a meeting with a group of women with the same struggles and issues and discussing sound stratagies for weight loss and healthy living helps me jump start my week. With a weekly weigh in and meeting I don't have a lot of fudge room to go "off the diet". Plus, getting lots of good tips for weight loss, cooking, eating, exercising, ect... helps put me in the frame of mind to stick to the diet.
4) They give out little stickers and everyone claps if you lose weight or exercise.
It may be cheesy, but the stickers and clapping really help.
5) I get to leave the house, no kids, buy a latte, and spend an hour and 1/2 with other girlfriends. It is like a mini vacation from life (or at least being a mom) each week.
They don't limit your weight loss per week, although they strongly discourage eating less than you are supposed to and losing too rapidly.
Unless you buy lots of their food products/cookbooks/ect... there are not any extra costs. I bought a $9 book that has points for a lot more foods than the one they give you for free, but I rarely use it. I mostly look up the points on 3FC or use the little slide rule.
A nice description of WW plans can be found here
http://www.3fatchicks.com/forum/showthread.php?t=55346