I often get asked to clarify how all of the objects in SharePoint relate to each. For example, people do not always understanding the differnence between a site collection and a site or how a web application and content database relate to each other.
To help out, I have produced this diagram which I think explains all of the main SharePoint objects and how they relate to each other. I did consider writing some kind of description to go with this, however I think it is better that I just do my best to answer any questions via the comments. If there are any re-occuring themes, I'll add to this article and try to explain them.
If you cannot see the image below, you can download it from here:
Image Updated to inlcude additional items from a reader's comment (thanks)