Winan,

There are a lot of differences, if I had to choose the most important it would be these:-

1] Grammar - C++ and C# have different language grammars, C# is based on the C++ language but its not C++ for example look at the difference between these examples.

C++ Example

#include "iostream"                 

using namespace std;

void main(void)
{
   cout << "Hello from Visual C++";
}

C# Example

using System;

class Sample {
   public static void Main()
   {
   Console.WriteLine("Hello from C#");
   }
}