To aid in the understanding of the structure, I am going to use a hard drive metaphor. In this scenario, the registry is your hard drive. The next level on your hard drive would be partitions on it. These are represented in the registry as subtrees. Furthering our metaphorical comparison, folders and subfolders are comparable to keys in the Registry.
Different keys represent different hardware or software objects.
There are over a thousand keys in the registry. Files in the folders would correspond to entries. Entries are what comprise the registry. Each setting in Windows is an entry in the registry. So this is what the registry would look like.