Difference Between web application, site collection and sites ?

Basically only difference between web application , site collection and site is understandable by SharePoint Hierarchy.

SharePoint Hierarchy:

To create site collection you need at least web application.

Site Collection:

• Site collection is logical container of multiple sites within a web application.
• Grouping sites in site collections allows those sites to share content, administrative settings, security rules, and, optionally, users and groups.

Top Level site:

• Top level site is also termed as site.
• Each Top Level Site has zero to many sub-sites (simply called sites)

Sites:

• Sites are simply inherit from site collection.
• These are treated as child's of site collection.
• A site can be a top-level site or a sub site of the top level site.
• A site can also have other sites – these are called sub-sites.