A SharePoint 2013 web application is composed of an Internet Information Services (IIS) web site that acts as a logical unit for the site collections that you create. Before you can create a site collection, we must first create a Web application. Each web application is represented by a different IIS web site with a unique or shared application pool. we can assign each web application a unique domain name. When you create a new web application, you also create a new content database and define the authentication method used to connect to the database. In addition, you define an authentication method to be used by the IIS Web site in SharePoint 2013.

• Server Farm
• Web Application
• Site Collections
• Sites
• Lists, Library..etc

Let’s start by navigation to the SharePoint Central Administration.here we will find under Application Management, Click on Mange web application link Button This page you can see which are already running web applications,now we have to create new web application then Click on New Button.  

• A pop-up window open for creating web application will appear Specify a port number.
• By default Allow Anonymous  selected No, if you your web application Anonymous site click yes.
• Select the authentication mode. By default, it will be Claims based authentication.
• We can change the content database name for easily identifying the database for your web application (eg: WSS_CONTENT_PortNumber).
• Click on OK button to start creating the web application

After Click ok Button It will take few Seconds to create web application Once the web application is created, the following pop-up will come and  here will be provide  to create a site Collection