I have been developing custom web parts and SharePoint customisations for a couple years now. During the early stages of SharePoint development I found a great deal of confusion when trying to retrieve information from different areas of an intranet through using the SPSite and SPWeb methods.
I think any other developer who is starting out SharePoint development may encounter the same issue. What I found useful was the “Site Architecture and Object Model Overview” diagram from the MSDN site.
It nicely breaks down the architecture of a SharePoint site. I highly recommend that you take look at the following links containing more diagrams:
Whilst I am talking about SPSite’s and SPWebs’s in this post, I’ll give you quick overview on how they work using the (above) diagram. Basically, the top level site collection (SPSite) contains your root web (SPWeb) and subsites (SPWebs under SPWebs). So, a subsite will be any site (SPWeb) under the top level web site in a site collection (SPSite).