| Web Application | Site Collection | Site(Sub Site) |
| Web Application is an IIS web site and it is on top position in SharePoint structure/hierarchy. | Site Collection, as the name means, is a collection of one or more sites. A Site collection will at least have one top level site and optionally sub sites created under that site. | Site is the component that contains the actual content like web parts, list, libraries, calendar, pages, etc... |
| While programming, SPWebApplication class represents Web Application. | While programming, SPSite class represents Site Collection. | While programming, SPWeb class represents Site. |
| A Web Application can have multiple Content Databases | 1) Per Site Collection, you can have maximum one Content Database.
Explanation: You can add multiple Content Databases in Web Application and while creating Site collection, use different Content Database in different Site Collection.
2) Multiple Site Collections can have one Content Database.
Explanation: You can have only one Content Database for Web Application and use the same while creating the Site Collections. | We cannot create separate Content Database for sub site. Content created in site goes to Site Collection Content Database only. |