What does lines with "using" means?

Question

In every C# program is it compulsory to use these lines?? and what does these lines means.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

Answer 1

Those are library linkages. These specify the compiler that, this program requires these libraries for compilation.

'using' is a term, just like #include in C.

Usually, for basic programs, only System is more than enough. You can leave Linq, Text and Tasks.
If you are creating or using List<>, then you will need Collections.

Answer 2

thanks buddy... :-)