There are four main kinds of applications: console applications, windows forms applications, web form applications (ASP.NET pages) and web services. Each of these has associated types.
For example, if you are developing a console application, you will need the type:
System.Console
And, if you are developing a windows forms application, you will find the following types useful:
System.Windows.Forms.Form System.Windows.Forms.Application System.Windows.Forms.Button System.Windows.Forms.TextBox System.Windows.Forms.Label System.EventArgs System.EventHandler System.Drawing.Point System.Drawing.Size System.Windows.Forms.MainMenu System.Windows.Forms.MenuItem
Later, we will look in more detail at console applications and windows forms applications.