There are many reasons a dog will bark and if you take some time you will notice that all the barks sound different
Attention Seeking
This is the most common type of bark, as dogs learn to use their voice to grab our attention. It may be a way of getting you to play or because they want feeding or are frustrated with something. These can also be just whines which sound similar to them being hurt.
As a greeting
Dogs barking when you arrive home is their way of saying ''HELLO''. This may be accompanied by whines, squeaks. Again this is very common and may start out a lot deeper in tone if they don't know who it is, then once they realise it is a friend and not foe, the tone will lighten.
When being territorial
When protecting their home or what they believe is their property a dog will bark. This is usually a deep bark normally accompanied with growls and body posturing which makes them look bigger. Some dogs may even have their hackles up.
In response to other dogs barking
If other dogs bark it normally sets off a chain reaction. There is a reason that the first dog barked so the rest of the dogs within a certain radius will respond in kind, be it in play or needing to protect something.
When frustrated
As mentioned earlier, dogs get frustrated, say you are doing a training session and they just aren't quite getting it. They will bark to show that they are getting frustrated and this will encourage you to try a different tactic.