Difference between SharePoint Web Application vs Site Collection vs Site vs Sub site

In technical terms, this is a web site in IIS (with an application pool associated). A web application needs to have at least 1 Site Collection. The Site Collection is the root site of the web site. Below the Site Collection, there can be one or more Sites. And a Site can contains sub sites.
An overview:
1. Web Application
1.1 Site Collection (SPSite)
1.1.1 Site (SPWeb)
1.1.2 Site (SPWeb)
1.2 Site Collection (SPSite)
1.2.1 Site (SPWeb)
1.2.1.1 Sub Site (SPWeb)
1.2.1.2 Sub Site (SPWeb)