Uh, they're two completely separate languages. C# is similar in structure to C++, but they're not interchangeable.
C++ is a more platform-independent standard application development language. That is, if you learn C++, you can write applications for linux, unix, windows, mac, etc... as long as you have a compiler for those systems.
C# is a purely Windows .NET development language. That means that if you learn C#, you can only write apps for Windows.
C# is a lot closer to scripting language than C++, so there's typically a shorter learning curve (it's easier to learn) and you can write more fully-functional applications faster, with less code. There's a lot of stuff that's just built-in to the language that you'd normally have to write yourself in C or C++.
As always, Google is your friend: