Copyright
Copyright is a way of declaring that you are the right owner of any type of creation such as a photo. The copyright office has a nice summary here. For photographers once you hit the shutter button that image is immediately considered copywriten. The reason to register you copyright is that it's just one more layer of protection against people using your photo without your permission. The process is not difficult - there is some expense involved but for images that are important to you it's not bad. You can assign copyright ownership to others - so if you want your children to be the heir to your copyright you may do that as well.