Agility is an absolutely fantastic sport for dogs, and for so many reasons. It's great physical and mental exercise, fun for you and your dog, and most of all it strengthens the bond between owner and dog. In order to be able to do agility, your dog has to learn how to pay attention to you and read the slightest of cues and respond to those cues in an instant. When competing, your dog is going through a new obstacle course and must look to you to know where to go and what to do. It takes hours and hours of training to get your dog used to all the different obstacles, but the process is extremely fun and completely worth it. I took a few agility courses with one of my dogs, he loved it and I noticed a great improvement in our everyday training afterwards.
There are a few things your dog must be good at before you even start the basics of agility. Having reliable recall and off leash behavior is a must, since agility is an off leash sport. Your dog must be able to be around other dogs and distractions and still listen to you.
You begin with the very basics in agility, usually just getting your dog used to the obstacles and using lots of treats and praise. But once your dog is familiar with what is required of him, you begin working on the most important part of agility, reading and responding to your ques. The finished product will be you running a little ahead of your dog and pointing to which obstacles you want him to do, while also saying their name. To get to this point you have to get your dog used to following your hands and knowing what each obstacle is called. For example, with the tunnel to you run with your dog and lead him into the tunnel with your hand at the same time that you say 'tunnel'. After a few times you run and wave him into the tunnel while you are a foot or two away, eventually you will be able to say tunnel and point from a long distance away and your dog will know what to do. This skill of looking to you for guidance and following your lead is something that will greatly improve all aspects of dog training, and help strengthen the bond between you and your dog.