What happen when you create Web application

When SharePoint creates a Web application the following actions happening behind the scene:

Web Ste:

• Creates a unique entry in SharePoint configuration DB for the Web App and assign GUID to that entry;
• Create and configures a Web application in IIS;
• Creates a root folder to store the Web application pages and associated resources;
• Creates and configures an IIS application pool;
• Configures authentication protocol and encryption settings;
• Assign a Default alternate access mapping for the Web app;
• Creates the first content database for the Web application;
• Associate a search service with the Web application;
• Assign a name to the Web application that appears in the Web application list in SharePoint Central Administration;
• Assign general settings to the Web application, such as maximum file upload size and default time zone;

Site Collection:

• Creates the top-level site based on a site definition;
• Sets general properties for the site, such as the site title and site owner;

Source from:http://sharepoint-sandbox.com