Hi.
You'll have to be a little more descriptive in your post. I don't know C#, but I do know that it uses standard programming syntax as seen in most mainstream programming languages. (Java, C++, C#, etc.)
sf:: is a namespace to SFML, which means that it holds a variety of classes and functions that the SFML library uses. You could call "using" at the head of your file instead of calling the namespace, but I wouldn't recommend it, considering namespaces are convenient for creating new functions with similar names.