Weight gaining
One of the most popular ways of training to put on lean mass is to have a bulk and cut cycle in your training. This will typically be split between the winter months for bulking and the summer months for cutting (so you look good on the beach :). Some people have trouble eating the amount of protein and carbohydrates that you will need to during there bulking cycle, if you don't eat enough food, then your not going to be able to bulk up properly. You will need to be eating a substancial ammount of carbs to fuel your training cycle, as well as a lot of protein to build yourself up again, ready for the next time you hit the gym.
If you are having trouble putting on the amount of weight that you want to, then instead of (or as well as in some cases) whey protein you should try taking a bulking powder. This typically contains a reasonable ammount of whey protein, as well as a large chunk of carbs and some fats. It's ideal for having forst thing in the morning if your in a rush, or drinking an hour or so before training. This will give you the energy you need to train properly. Imediatly after training is also good as it replaces the fuel that you have burned up.
Weight gainer is not somthing that you would normally be taking whilse trying to cut, and I always switch back to whey protein if I've been on weight gainer.
So there you have it, it's a great, easy, inexpensive way to help you put on some mass whilst youe in your bulking cycle. It's inevitable that you will also gain a little fat whilst you are doing this, but you should be able to get it off easily enough when you start cutting again.