This is a hub. As you can see, it has bearings and other components to hold the bearings in place. Newer axles have sealed bearing that do not have loose parts. In the picture below I have identified the fact that spokes that are one hole apart on the hub are 4 holes apart on the rim. In the process of building the wheel the spoke crosses over 3 other spokes and this pattern is repeated throughout the rim. In all this rim has 36 spokes. Review the wheel building article by Sheldon Brown to learn more about wheel building.