This question already has an answer here:
I've seen both when looking up P/Invoke definitions... sometimes [Out] is used, and sometimes out... I'm assuming they're the same.
They are the equivalent to each other when used in the context of P/Invoke - see OutAttribute.
You can apply the OutAttribute to value and reference types passed by reference to change In/Out behavior to Out-only behavior, which is equivalent to using the out keyword in C#.
In addition to James answer, note that the out contextual keyword is used in two contexts:
As a parameter modifier in parameter lists
In generic type parameter declarations in interfaces and delegates
Source: http://msdn.microsoft.com/en-us/library/t3c3bfhx.aspx
