A UCM component is defined as a directory and everything recursively underneath that directory.
Sometimes the root directory of a component is also the root of a VOB, but this is not used much except in the very early days of UCM where a 1:1 relation between a component and a VOB was not optional. Today this option is kept solely for backwards compatibility.
In general you'll find that a component root is the 2nd level directory of a VOB. And consequently the 2nd level of UCM Client VOB is always a component root. There is simply no other way that you can create a directory at this level - except to create a new component.
Besides hosting all the version controlled elements (files and directories) UCM components also have the very distinguished purpose to hold or contain baselines.
A special variant of components actually have this ability to contain baselines as their sole purpose. These components are rootless components. These rootless components hold baselines that references other baselines and through this composite structure they make up composite baselines.