Dog training is the process of teaching a dog to perform behaviors in response to certain commands. There are many type of dog training and many objectives, from basic obedience training to specialized areas including police dog, search and rescue, assistance to people with disabilities and entertainment.
The fact that people often forget that dogs do not understand every word they are saying, make dog training an essential thing to get! Just keep in your mind that they do not have a memory that functions like ours. I give you an example: he will not understand, if you have been out and left your dog alone, then arrive home to find he has messed on the floor. It is no good at all by shouting at your dog or punishing him, since they cannot connect between what you are shouting about now and their messy actions of few minutes ago.
In order that your dog understands a reprimand it has to be done at the time that the undesirable action is occurring in order for the dog to understand. It may be frustrating that your dog may do the undesirable actions a few times before you catch him in the act to deal with it, you must have patience and persevere, do not give in and punish after the event because it is pointless and will be ineffective, and in actual fact may make matters much worse.
You should obviously ask yourself why your dog is doing the undesirable behavior too, for example he may need a good walk before you leave him so he has been to the toilet and is a bit sleepy and may then sleep whilst you are out.