Gaining weight (on the nutrition side) is mainly about eating more calories than you burn. Unfortunately it's sometimes hard to get all those calories into your daily meals. Eating the right kinds of snacks will make it easier for you to reach your daily calorie goal.
All foods, including snacks, derive their calories from three types of nutrients: fats, carbohydrates and proteins. To gain weight, you need to eat the types of foods that will give you energy to lift heavy weights and provide the essential building blocks to repair the damage you've done to your muscles. The ideal snacks will give you you carbohydrates for energy, protein to repair and will taste good so you can eat them.
Bodybuilders are in love with tuna. Some eat more than one can per day. A 6oz can contains 150 calories which is almost all from protein (30 grams of it). This is a great daytime or nighttime snack: light on calories, high in protein. If you want it to be very low in fat, eat tuna in water instead of in oil.
Dairy products are great snacks for building muscle. Like tuna, dairy comes from an animal which means it is a complete protein.
Cottage cheese and milk (regular or chocolate) are good choices. They come in whole, low fat or fat free varieties and contain about 8 grams of protein per serving. Since these are part liquid, they're a lot easier to eat than whole foods which can help you increase your daily calorie intake.
Nuts are also a good muscle building snack. A 1/4 cup of nuts contains 160 calories. They are high in fat but almost all of it is unsaturated (the healthy kind).
Fruits and Vegetables
Fruits and vegetables are often overlooked by people who want to gain weight. They contain little calories and almost no protein. So if you're trying to gain weight, what's the point of eating them? The point is that they have a lot of nutrients - vitamins, minerals and fiber. For you to be healthy and be able to gain weight in a healthy way, you need to get all these nutrients which means eating your fruits and veggies.
If you like the more traditional snack foods (chocolate, chips, pretzels) don't be afraid to eat them. Since you're gaining weight, too many calories shouldn't be a huge concern.
The Bottom Line
Snacks can help you increase your daily calorie intake without too much effort. A snack takes a few seconds to prepare and a minute to eat. Don't leave them out of your diet as they can easily add up to 400 or more calories a day. If you're serious about gaining weight, snacks should be one of your priorities.