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#");
}
}