Weight training alone can be used to loose weight but there are a couple things to consider if you want to do a weight training only fitness program.

1.  Depending on the duration and frequency of weight training you can actually gain weight while losing fat.

2.  It is more difficult to lose weight -if that is your only goal- using a weight training only program unless you do circuit training which incorporates both weight training and a cardio program at the same time.  This means you have to keep your heart rate between 65% to 80% max for the duration of the workout.  I have found this nearly impossilbe to do due to the layout and amount of people in most of the gyms I have worked out in.

3.  A good cardiovascular program is a must for a well rounded fitness program.  Usually keeping your heart in the 65% to 80% of maximum for a mininum of 30 minutes three times a week is sufficient.

4.  A weight training program is helpful in losing weight.  As you add more lean muscle mass to your body your metabolism will rise causing you to burn more calories while at rest.

In conclusion, I do a combintion of both weight training and cardio alternating days and taking the weekend off.  Hope this helps.