Most photographers end up owning several Camera Shoulder Bag. This is because a camera bag is not an all-purpose holdall for containing ones entire collection of equipment, but a transport medium for the equipment necessary for a particular shoot, and the requirements vary from shoot to shoot.
In essence, a bag must:
Contain all the equipment you need for the shoot
Protect the equipment against the likely shocks travel and shooting will generate
Protect the equipment against likely weather conditions
Be carryable with the equipment required over distance to be walked
Be stowable in whatever transport is to be used
Allow for easy identification and retrieval of components needed
There are four basic types of camera bags and cases:
Single camera bags, often shaped like the camera and lens
Flight cases, either metal (typically aluminium) or synthetic, such as Pelicases
With the exception of single camera bags, which are generally bought with the camera/lens combination, these types come in a variety of sizes, from a bag which can hold a camera, one lens and a flashgun, to a bag which can hold two deep Pro bodies and a variety of pro lenses. Bags of this kind quickly become too heavy to carry any distance, and may cause back problems.
Camera rucksacks are the easiest way of carrying substantial loads over distance, but are the least convenient for accessing equipment.
Single camera bags are the most convenient for accessing the camera, but attract thieves and have no flexibility for different equipment combinations.
Flight cases are the most convenient for accessing equipment, but the most difficult to carry over any distance. They are the only type suitable for stowage in an aircraft hold
Shoulder-type bags are the most convenient for small systems, such as a camera and two lenses, but heavy shoulder-type bags are the most damaging to the carriers musculo-skeletal system
Lowepro Camera Shoulder Bag and others manufacture bags which are a hybrid between rucksack and shoulder bag.
Other issues to consider when choosing a bag: