What are they?
There are different kinds of protection for SLR cameras. The main two are bags and cases. They each serve their different purposes for different photographers.
A camera bag has at least one main pocket, sometimes having multiple compartments, for storage of lenses and a flash or something. Most bags also have outside pockets. These are to keep your extra batteries, lens caps, remotes, filters and other things that you do not necessarily use all of the time, but are good to have with you.
On the other hand, cases are more along the lines of one camera and one lens protection. Some of them have a pocket, but usually they are simply just placed around the camera. The one that I used to have you simply laced the elastic band around the lens, and slid the camera in. It was actually quite nice to have because the case protected it, but it was also attached to the camera, so I never had to worry about losing it.
Which one works for whom?
First I will discuss camera bags. If you are a beginning photographer who is looking to become more involved in photography, or if you are already intensely involved in photography, I would probably recommend a bag. The reason for this is that you may want to take a step up as far as more lenses and accessories. Bags are also good for when you are traveling. The reason being that if you go somewhere, you can put all of your charges, and memory cards and such into one bag and not have to worry about keeping track of everything. Bags are the best thing for beginners looking to further their interest and immersion in photography. However, if you can I would also recommend that when you first start, you get a case.
The reason cases are good is that they keep your camera in great shape. There is no room for movement, and it is perfect for beginners. Even though camera cases are not that practical for those of us with multiple lenses, and those of us with many cards and filters and batteries, they make a great extra protection to have. For example, if I am going somewhere where I only need one lens, I take my case, because it holds my camera and keeps it safe and still fits around my neck. Cases are just simple and easy fixes to make sure that your camera remains safe, no matter where you are.
Which one to buy?
As far as bags go, I would recommend one with, at the very least, a main pocket and one side pocket. I would say that that is the minimum criteria for any bag. However, as you start growing in the size and number of lenses, you will want to get a bag with a little more space. I personally would recommend a backpack. They are easier to carry, and hold up very nicely. If you are a beginning photographer with one lens and a couple accessories there is no need for a huge backpack yet. You can last with a simple shoulder strap bag. However, for the amateur or pro photographers, it is almost vital to get a backpack, or something along those lines. There are plenty of shoulder strap bags that would work for pro photographers, and some of them even have more space than the backpacks. But, for me I like to have the weight of my whole camera bag on both shoulders, rather than just one.
For cases, I would recommend a neoprene camera case. The reason is that it prevents scratches without adding much weight to the camera. Also, I would recommend getting one with at least something that can attach to your camera. There is nothing worse than buying a case and losing it because you had to take it off your camera quickly and snap a picture. The good thing about having the case attached to your camera is that you do not lose it, and it usually helps the camera slide in easier.
REMEMBER, whether getting a camera case or a camera bag, or any of the other things that can be used to protect your camera, higher price does not mean better quality. Make sure you find something inside your price range, and I am sure that you can find something that is inexpensive and still works perfectly.