What is use of partial keyword?

partial keyword allows you to split the definition of one class into multiple files.
For Example: When you open a project the default form has following defintion:
 public partial class Form1 : Form

this allow the definition of class Form1 to split into multiple files  i.e.  Form1.cs and Form1.Designer.cs
hence the members of Form1 can be used in Form1.Designer.cs