ref out

They're pretty much the same - the only difference is that a variable you pass as an out parameter doesn't need to be initialized but passing it as a ref parameter it has to be set to something.

int x;
Foo(out x); // OK

int y;
Foo(ref y); // Error: y should be initialized before calling the method

Ref parameters are for data that might be modified, out parameters are for data that's an additional output for the function (eg int.TryParse) that are already using the return value for something.