Backpacks and sling bags are used to transport materials in a sack that hangs on a person's body with straps. Backpacks are bigger and usually more durable than sling bags. But sling bags are usually considered more fashionable than backpacks; when worn by men, they are sometimes called "man purses."
Backpacks are used to carry materials on the back. Sometimes a backpack is called a "book bag" because it is used by students to carry their textbooks and school supplies from one class to another. Sling bags are usually too frail for multiple textbooks, but they can be used for carrying supplies as well. Some sling bags are called "messenger bags," but the difference is actually one of size. Most messenger bags are quite large, unlike most sling bags.
Backpacks have two straps on the back of the bag—one for each shoulder. Sling bags only have one strap. In both cases, the straps are adjustable. The internal compartments of backpacks are usually accessed by zippers, where there are usually many other pockets for smaller items. Sling bags typically have a large flap that folds over the front of the bag and attaches by a button at the bag's bottom. Sling bags are usually easier to get into while being worn, unlike backpacks, which are better suited for hikes and uses that don't involve a need to retrieve items frequently from the pack.
Backpacks differ mostly in size. They are designed more for function than fashion, which is why they are often used for camping and military trips. Sling bags have many shapes, styles and designs. Sometimes the straps are very long and cross the chest so that the bag hangs below the opposite shoulder from where the strap hangs. Some sling bags are designed for specific purposes, such as laptop or camera cases. A laptop sling bag will fit the dimensions of a typical laptop, a thin rectangular shape. A camera sling bag will have a more boxy shape.
The easiest way to identify a backpack and sling bag is to look at how many straps are on the bag. Even though a backpack can be worn like a sling bag, using only one of the straps, simply by having two straps it is considered a backpack. A sling will never have two straps. By definition, a "sling" is a single strap.
The main question you have to ask yourself when deciding whether to buy a backpack or sling bag is what you find more important: fashion or function? If you want your bag to be seen, find a stylish sling bag. But if you're going on an outdoor adventure where it doesn't matter what you look like, a backpack is the better choice.